Golf ball

ABSTRACT

A golf ball  2  includes a spherical core  4  and a cover  6  covering the core  4 . The cover  6  includes an inner cover  8  and an outer cover  10  positioned outside the inner cover  8 . The core  4  is obtained by a rubber composition being crosslinked. The rubber composition includes a base rubber (a), a co-crosslinking agent (b), a crosslinking initiator (c), and a carboxylate (d). The co-crosslinking agent (b) is:
         (b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; or   (b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.
 
The amount of the carboxylate (d) is equal to or greater than 1 parts by weight but less than 40 parts by weight per 100 parts by weight of the base rubber (a). A hardness of an innermost layer of the cover is greater than a surface hardness of the core.

This application claims priority on Patent Application No. 2011-145379filed in JAPAN on Jun. 30, 2011. The entire contents of this JapanesePatent Application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to golf balls. Specifically, the presentinvention relates to golf balls that include a solid core and a coverhaving two or more layers.

2. Description of the Related Art

Golf players' foremost requirement for golf balls is flight performance.In particular, golf players place importance on flight performance uponshots with a driver and a long iron. Flight performance correlates withthe resilience performance of a golf ball. When a golf ball havingexcellent resilience performance is hit, the golf ball flies at a highspeed, thereby achieving a large flight distance. Golf balls thatinclude a core having excellent resilience performance are disclosed inJPS61-37178, JP2008-212681 (US2008/0214324), JP2008-523952(US2006/0135287 and US2007/0173607), and JP2009-119256 (US2009/0124757).

The core disclosed in JPS61-37178 is obtained from a rubber compositionthat includes a co-crosslinking agent and a crosslinking activator. Thispublication discloses palmitic acid, stearic acid, and myristic acid asthe crosslinking activator.

The core disclosed in JP2008-212681 is obtained from a rubbercomposition that includes an organic peroxide, a metal salt of anα,β-unsaturated carboxylic acid, and a copper salt of a fatty acid.

The core disclosed in JP2008-523952 is obtained from a rubbercomposition that includes a metal salt of an unsaturated monocarboxylicacid, a free radical initiator, and a non-conjugated diene monomer.

The core disclosed in JP2009-119256 is obtained from a rubbercomposition that includes a polybutadiene whose vinyl content is equalto or less than 2%, whose cis 1,4-bond content is equal to or greaterthan 80%, and which has an active end modified with an alkoxysilanecompound.

An appropriate trajectory height is required in order to achieve a largeflight distance. A trajectory height depends on a spin rate and a launchangle. In a golf ball that achieves a high trajectory by a high spinrate, a flight distance is insufficient. In a golf ball that achieves ahigh trajectory by a high launch angle, a large flight distance isobtained. Use of an outer-hard/inner-soft structure in a golf ball canachieve a low spin rate and a high launch angle. Modifications regardinga hardness distribution of a core are disclosed in JPH6-154357 (U.S.Pat. No. 5,403,010), JP2008-194471 (U.S. Pat. No. 7,344,455,US2008/0194358, US2008/0194359, and US2008/0214325), and JP2008-194473(US2008/0194357 and US2008/0312008).

In the core disclosed in JPH6-154357, a JIS-C hardness H1 at the centralpoint of the core is 58 to 73, a JIS-C hardness H2 in a region thatextends over a distance range from equal to or greater than 5 mm toequal to or less than 10 mm from the central point of the core is equalto or greater than 65 but equal to or less than 75, a JIS-C hardness H3at a point located at a distance of 15 mm from the central point isequal to or greater than 74 but equal to or less than 82, and a JIS-Chardness H4 at the surface of the core is equal to or greater than 76but equal to or less than 84. The hardness H2 is greater than thehardness H1, the hardness H3 is greater than the hardness H2, and thehardness H4 is equal to or greater than the hardness H3.

In the core disclosed in JP2008-194471, a Shore D hardness at thecentral point of the core is equal to or greater than 30 but equal to orless than 48, a Shore D hardness at a point located at a distance of 4mm from the central point is equal to or greater than 34 but equal to orless than 52, a Shore D hardness at a point located at a distance of 8mm from the central point is equal to or greater than 40 but equal to orless than 58, a Shore D hardness at a point located at a distance of 12mm from the central point is equal to or greater than 43 but equal to orless than 61, a Shore D hardness in a region that extends over adistance range from equal to or greater than 2 mm to equal to or lessthan 3 mm from the surface of the core is equal to or greater than 36but equal to or less than 54, and a Shore D hardness at the surface ofthe core is equal to or greater than 41 but equal to or less than 59.

In the core disclosed in JP2008-194473, a Shore D hardness at thecentral point of the core is equal to or greater than 25 but equal to orless than 45, a Shore D hardness in a region that extends over adistance range from equal to or greater than 5 mm to equal to or lessthan 10 mm from the central point is equal to or greater than 39 butequal to or less than 58, a Shore D hardness at a point located at adistance of 15 mm from the central point is equal to or greater than 36but equal to or less than 55, and a Shore D hardness at the surface ofthe core is equal to or greater than 55 but equal to or less than 75.

JP2010-253268 (US2010/0273575) discloses a golf ball that includes acore, an envelope layer, a mid layer, and a cover. In the core, thehardness gradually increases from the central point of the core to thesurface of the core. The difference between a JIS-C hardness at thesurface and a JIS-C hardness at the central point is equal to or greaterthan 15. The hardness of the cover is greater than the hardness of themid layer, and the hardness of the mid layer is greater than thehardness of the envelope layer.

For a tee shot on a par-three hole and a second shot on a par-four hole,a middle iron is frequently used. Golf players also desire a largeflight distance upon a shot with a middle iron. An object of the presentinvention is to provide a golf ball that has excellent flightperformance upon a shot with a middle iron.

SUMMARY OF THE INVENTION

A golf ball according to the present invention comprises a sphericalcore and a cover which covers the core and has two or more layers. Thecore is obtained by a rubber composition being crosslinked. The rubbercomposition includes:

(a) a base rubber;

(b) a co-crosslinking agent;

(c) a crosslinking initiator; and

(d) a carboxylate.

The co-crosslinking agent (b) is:

(b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; or

(b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms.

An amount of the carboxylate (d) is equal to or greater than 1 parts byweight but less than 40 parts by weight per 100 parts by weight of thebase rubber (a). A JIS-C hardness Hi of an innermost layer of the coveris greater than a JIS-C hardness Hs at a surface of the core.

In the golf ball according to the present invention, a hardnessdistribution is appropriate. In the golf ball, the energy loss is lowwhen being hit with a middle iron. The golf ball has excellentresilience performance. When the golf ball is hit with a middle iron,the spin rate is low. The great resilience performance and the low spinrate achieve a large flight distance.

Preferably, the hardness Hi is equal to or greater than 80.0 but equalto or less than 95.0. Preferably, the hardness Hs is equal to or greaterthan 78.0 but equal to or less than 95.0.

Preferably, the rubber composition further includes an organic sulfurcompound (e).

When the rubber composition includes the α,β-unsaturated carboxylic acid(b1), the rubber composition further includes a metal compound (f).

Preferably, the carboxylate (d) is a fatty acid salt. Preferably, thecarboxylate (d) is a salt of a saturated fatty acid. Preferably, acarbon number of a carboxylic acid component of the carboxylate (d) isequal to or greater than 4 but equal to or less than 30.

Preferably, the rubber composition includes the metal salt (b2) of theα,β-unsaturated carboxylic acid.

Preferably, the organic sulfur compound (e) is a thiophenol, apolysulfide having 2 to 4 sulfur atoms, a thionaphthol or a thiuram, ora metal salt thereof.

Preferably, the rubber composition includes 15 parts by weight orgreater but 50 parts by weight or less of the co-crosslinking agent (b)per 100 parts by weight of the base rubber (a). Preferably, the rubbercomposition includes 0.2 parts by weight or greater but 5.0 parts byweight or less of the crosslinking initiator (c) per 100 parts by weightof the base rubber (a). Preferably, the rubber composition includes 0.05parts by weight or greater but 5 parts by weight or less of the organicsulfur compound (e) per 100 parts by weight of the base rubber (a).

Preferably, a difference (Hs−H(0)) between the hardness Hs and a JIS-Chardness H(0) at a central point of the core is equal to or greater than15. Preferably, a JIS-C hardness H(0) at a central point of the core isequal to or greater than 40.0 but equal to or less than 70.0.

Preferably, a difference (Hi−Hs) between the hardness Hi and thehardness Hs is equal to or greater than 1 but equal to or less than 5.Preferably, a difference (Ho−Hi) between a JIS-C hardness Ho of anoutermost layer of the cover and the JIS-C hardness Hi of the innermostlayer of the cover is equal to or greater than 2 but equal to or lessthan 10.

Preferably, in a hardness distribution curve from a central point of thecore to an outermost layer of the cover, a hardness of the outermostlayer is the greatest.

Preferably, a total thickness of the cover is equal to or less than 2.5mm.

When distances (mm) from a central point of the core to the surface ofthe core and eight points, and JIS-C hardnesses at the surface of thecore and the eight points, which eight points are obtained by dividing aregion from the central point of the core to the surface of the core atintervals of 2.5 mm, are plotted in a graph, R² of a linearapproximation curve obtained by a least-square method is preferablyequal to or greater than 0.95.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway cross-sectional view of a golf ballaccording to one embodiment of the present invention; and

FIG. 2 is a line graph showing a hardness distribution of a core of thegolf ball in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe in detail the present invention, based onpreferred embodiments with reference to the accompanying drawings.

A golf ball 2 shown in FIG. 1 includes a spherical core 4 and a cover 6covering the core 4. The cover 6 includes an inner cover 8 and an outercover 10 positioned outside the inner cover 8. The inner cover 8 is aninnermost layer of the cover 6. The outer cover 10 is an outermost layerof the cover 6. The cover 6 may include another one or more layersbetween the inner cover 8 and the outer cover 10. On the surface of theouter cover 10, a large number of dimples 12 are formed. Of the surfaceof the golf ball 2, a part other than the dimples 12 is a land 14. Thegolf ball 2 includes a paint layer and a mark layer on the external sideof the outer cover 10 although these layers are not shown in thedrawing.

The golf ball 2 has a diameter of 40 mm or greater but 45 mm or less.From the standpoint of conformity to the rules established by the UnitedStates Golf Association (USGA), the diameter is preferably equal to orgreater than 42.67 mm. In light of suppression of air resistance, thediameter is preferably equal to or less than 44 mm and more preferablyequal to or less than 42.80 mm. The golf ball 2 has a weight of 40 g orgreater but 50 g or less. In light of attainment of great inertia, theweight is preferably equal to or greater than 44 g and more preferablyequal to or greater than 45.00 g. From the standpoint of conformity tothe rules established by the USGA, the weight is preferably equal to orless than 45.93 g.

The core 4 is obtained by crosslinking a rubber composition. The rubbercomposition includes:

(a) a base rubber;

(b) a co-crosslinking agent;

(c) a crosslinking initiator; and

(d) a carboxylate.

During heating and forming of the core 4, the base rubber (a) iscrosslinked by the co-crosslinking agent (b). The heat of thecrosslinking reaction remains near the central point of the core 4.Thus, during heating and forming of the core 4, the temperature at thecentral portion is high. The temperature gradually decreases from thecentral point toward the surface. The carboxylate (d) reacts with ametal salt of the co-crosslinking agent (b) to exchange cation. Thisexchange reaction is likely to take place in the central portion of thecore 4 where the temperature is high, and is unlikely to take place nearthe surface of the core 4. In other words, breaking of metal crosslinksis likely to occur in the central portion of the core 4 and is unlikelyto occur near the surface of the core 4. As a result, the crosslinkingdensity of the core 4 increases from its central point toward itssurface. The core 4 can achieve an outer-hard/inner-soft structure.Further, when the rubber composition includes an organic sulfur compound(e) together with the carboxylate (d), the gradient of a hardnessdistribution can be controlled, and the degree of theouter-hard/inner-soft structure of the core 4 can be increased. When thegolf ball 2 that includes the core 4 is hit with a middle iron, the spinrate is low. The golf ball 2 achieves excellent flight performance upona shot with a middle iron.

FIG. 2 is a line graph showing a hardness distribution of the core 4 ofthe golf ball 2 in FIG. 1. The horizontal axis of the graph indicates adistance (mm) from the central point of the core 4. The vertical axis ofthe graph indicates a JIS-C hardness. Eight measuring points obtained bydividing a region from the central point of the core 4 to the surface ofthe core 4 at intervals of 2.5 mm are plotted in the graph. A measuringpoint on the surface of the core 4 is also plotted in the graph. Thedistance from the central point of the core 4 to each of these measuringpoints is as follows.

First point: 0.0 mm (central point)

Second point: 2.5 mm

Third point: 5.0 mm

Fourth point: 7.5 mm

Fifth point: 10.0 mm

Sixth point: 12.5 mm

Seventh point: 15.0 mm

Eighth point: 17.5 mm

Ninth point: surface

Hardnesses at the first to eighth points are measured by pressing aJIS-C type hardness scale against a cut plane of the core 4 that hasbeen cut into two halves. A hardness Hs at the ninth point is measuredby pressing the JIS-C type hardness scale against the surface of thecore 4. For the measurement, an automated rubber hardness measurementmachine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), towhich this hardness scale is mounted, is used. In the present invention,a JIS-C hardness at a measuring point whose distance from the centralpoint of the core 4 is x mm is represented by H(x). A hardness at thecentral point of the core 4 is represented by H(0).

FIG. 2 also shows a linear approximation curve obtained by aleast-square method on the basis of the distances and the hardnesses ofthe nine measuring points. As is clear from FIG. 2, the broken line doesnot greatly deviate from the linear approximation curve. In other words,the broken line has a shape close to the linear approximation curve. Inthe core 4, the hardness linearly increases from its central point toits surface. When the core 4 is hit with a middle iron, the energy lossis low. The core 4 has excellent resilience performance. When the golfball 2 is hit with a middle iron, the flight distance is large.

In the core 4, R² of the linear approximation curve obtained by theleast-square method is equal to or greater than 0.95. R² is an indexindicating the linearity of the broken line. For the core 4 for which R²is equal to or greater than 0.95, the shape of the broken line of thehardness distribution is close to a straight line. The core 4 for whichR² is equal to or greater than 0.95 has excellent resilienceperformance. R² is more preferably equal to or greater than 0.96 andparticularly preferably equal to or greater than 0.97. R² is calculatedby squaring a correlation coefficient R. The correlation coefficient Ris calculated by dividing the covariance of the distance (%) from thecentral point and the hardness (JIS-C) by the standard deviation of thedistance (%) from the central point and the standard deviation of thehardness (JIS-C).

Examples of the base rubber (a) of the core 4 include polybutadienes,polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-dienecopolymers, and natural rubbers. In light of resilience performance,polybutadienes are preferred. When a polybutadiene and another rubberare used in combination, it is preferred that the polybutadiene isincluded as a principal component. Specifically, the proportion of thepolybutadiene to the entire base rubber is preferably equal to orgreater than 50% by weight and more preferably equal to or greater than80% by weight. The proportion of cis-1,4 bonds in the polybutadiene ispreferably equal to or greater than 40% by weight and more preferablyequal to or greater than 80% by weight.

A polybutadiene in which the proportion of 1,2-vinyl bonds is equal toor less than 2.0% by weight is preferred. The polybutadiene cancontribute to the resilience performance of the core 4. In this respect,the proportion of 1,2-vinyl bonds is preferably equal to or less than1.7% by weight and particularly preferably equal to or less than 1.5% byweight.

From the standpoint that a polybutadiene having a low proportion of1,2-vinyl bonds and excellent polymerization activity is obtained, it ispreferred that a rare-earth-element-containing catalyst is used forsynthesizing a polybutadiene. In particular, a polybutadiene synthesizedwith a catalyst containing neodymium, which is a lanthanum-series rareearth element compound, is preferred.

The polybutadiene has a Mooney viscosity (ML₁₊₄(100° C.)) of preferably30 or greater, more preferably 32 or greater, and particularlypreferably 35 or greater. The Mooney viscosity (ML₁₊₄(100° C.)) ispreferably equal to or less than 140, more preferably equal to or lessthan 120, even more preferably equal to or less than 100, andparticularly preferably equal to or less than 80. The Mooney viscosity(ML₁₊₄(100° C.)) is measured according to the standards of “JIS K6300”.The measurement conditions are as follows.

Rotor: L rotor

Preheating time: 1 minute

Rotating time of rotor: 4 minutes

Temperature: 100° C.

In light of workability, the polybutadiene has a molecular weightdistribution (Mw/Mn) of preferably 2.0 or greater, more preferably 2.2or greater, even more preferably 2.4 or greater, and particularlypreferably 2.6 or greater. In light of resilience performance, themolecular weight distribution (Mw/Mn) is preferably equal to or lessthan 6.0, more preferably equal to or less than 5.0, even morepreferably equal to or less than 4.0, and particularly preferably equalto or less than 3.4. The molecular weight distribution (Mw/Mn) iscalculated by dividing the weight average molecular weight Mw by thenumber average molecular weight Mn.

The molecular weight distribution is measured by gel permeationchromatography (“HLC-8120GPC” manufactured by Tosoh Corporation). Themeasurement conditions are as follows.

Detector: differential refractometer

Column: GMHHXL (manufactured by Tosoh Corporation)

Column temperature: 40° C.

Mobile phase: tetrahydrofuran

The molecular weight distribution is calculated as a value obtained byconversion using polystyrene standard.

The co-crosslinking agent (b) is:

(b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; or

(b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms.

The rubber composition may include only the α,β-unsaturated carboxylicacid (b1) or only the metal salt (b2) of the α,β-unsaturated carboxylicacid as the co-crosslinking agent (b). The rubber composition mayinclude both the α,β-unsaturated carboxylic acid (b1) and the metal salt(b2) of the α,β-unsaturated carboxylic acid as the co-crosslinking agent(b).

The metal salt (b2) of the α,β-unsaturated carboxylic acidgraft-polymerizes with the molecular chain of the base rubber, therebycrosslinking the rubber molecules. When the rubber composition includesthe α,β-unsaturated carboxylic acid (b1), the rubber compositionpreferably further includes a metal compound (f). The metal compound (f)reacts with the α,β-unsaturated carboxylic acid (b1) in the rubbercomposition. A salt obtained by this reaction graft-polymerizes with themolecular chain of the base rubber.

Examples of the metal compound (f) include metal hydroxides such asmagnesium hydroxide, zinc hydroxide, calcium hydroxide, sodiumhydroxide, lithium hydroxide, potassium hydroxide, and copper hydroxide;metal oxides such as magnesium oxide, calcium oxide, zinc oxide, andcopper oxide; and metal carbonates such as magnesium carbonate, zinccarbonate, calcium carbonate, sodium carbonate, lithium carbonate, andpotassium carbonate. A compound that includes a bivalent metal ispreferred. The compound that includes the bivalent metal reacts with theco-crosslinking agent (b) to form metal crosslinks. The metal compound(f) is particularly preferably a zinc compound. Two or more metalcompounds may be used in combination.

Examples of the α,β-unsaturated carboxylic acids include acrylic acid,methacrylic acid, fumaric acid, maleic acid, and crotonic acid. Examplesof the metal component in the metal salt (b2) of the α,β-unsaturatedcarboxylic acid include sodium ion, potassium ion, lithium ion,magnesium ion, calcium ion, zinc ion, barium ion, cadmium ion, aluminumion, tin ion, and zirconium ion. The metal salt (b2) of theα,β-unsaturated carboxylic acid may include two or more types of ions.From the standpoint that metal crosslinks are likely to occur betweenthe rubber molecules, bivalent metal ions such as magnesium ion, calciumion, zinc ion, barium ion, and cadmium ion are preferred. The metal salt(b2) of the α,β-unsaturated carboxylic acid is particularly preferablyzinc acrylate.

In light of resilience performance of the golf ball 2, the amount of theco-crosslinking agent (b) is preferably equal to or greater than 15parts by weight and particularly preferably equal to or greater than 20parts by weight, per 100 parts by weight of the base rubber. In light offeel at impact, the amount is preferably equal to or less than 50 partsby weight, more preferably equal to or less than 45 parts by weight, andparticularly preferably equal to or less than 40 parts by weight, per100 parts by weight of the base rubber.

The crosslinking initiator (c) is preferably an organic peroxide. Theorganic peroxide contributes to the resilience performance of the golfball 2. Examples of preferable organic peroxides include dicumylperoxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. Inlight of versatility, dicumyl peroxide is preferred.

In light of resilience performance of the golf ball 2, the amount of thecrosslinking initiator (c) is preferably equal to or greater than 0.2parts by weight and particularly preferably equal to or greater than 0.5parts by weight, per 100 parts by weight of the base rubber. In light offeel at impact and durability of the golf ball 2, the amount ispreferably equal to or less than 5.0 parts by weight and particularlypreferably equal to or less than 2.5 parts by weight, per 100 parts byweight of the base rubber.

The co-crosslinking agent (b) is not included in the carboxylate (d).The carboxylic acid component of the carboxylate (d) has a carboxylgroup. The carboxylate (d) exchanges a cationic component with theco-crosslinking agent (b). It is inferred that the carboxylate (d)breaks the metal crosslinks by the co-crosslinking agent (b) at thecentral portion of the core 4 during heating and forming of the core 4.

The carbon number of the carboxylic acid component of the carboxylate(d) is preferably equal to or greater than 4 but equal to or less than30, is more preferably equal to or greater than 9 but equal to or lessthan 30, and is particularly preferably equal to or greater than 14 butequal to or less than 28. Examples of the carboxylic acid includealiphatic carboxylic acids (fatty acids) and aromatic carboxylic acids.Salts of fatty acids are preferred.

The rubber composition may include a salt of a saturated fatty acid ormay include a salt of an unsaturated fatty acid. The salt of thesaturated fatty acid is preferred.

Examples of fatty acids include butyric acid (C4), valeric acid (C5),caproic acid (C6), enanthic acid (C7), caprylic acid (C8), pelargonicacid (C9), capric acid (C10), lauric acid (C12), myristic acid (C14),myristoleic acid (C14), pentadecylic acid (C15), palmitic acid (C16),palmitoleic acid (C16), margaric acid (C17), stearic acid (C18), elaidicacid (C18), vaccenic acid (C18), oleic acid (C18), linolic acid (C18),linolenic acid (C18), 12-hydroxystearic acid (C18), arachidic acid(C20), gadoleic acid (C20), arachidonic acid (C20), eicosenoic acid(C20), behenic acid (C22), erucic acid (C22), lignoceric acid (C24),nervonic acid (C24), cerotic acid (C26), montanic acid (C28), andmelissic acid (C30). Two or more fatty acid salts may be used incombination. Salts of myristic acid, palmitic acid, stearic acid,behenic acid, and oleic acid are preferred.

An aromatic carboxylic acid has an aromatic ring and a carboxyl group.Examples of aromatic carboxylic acids include benzoic acid, phthalicacid, isophthalic acid, terephthalic acid, hemimellitic acid(benzene-1,2,3-tricarboxylic acid), trimellitic acid(benzene-1,2,4-tricarboxylic acid), trimesic acid(benzene-1,3,5-tricarboxylic acid), mellophanic acid(benzene-1,2,3,4-tetracarboxylic acid), prehnitic acid(benzene-1,2,3,5-tetracarboxylic acid), pyromellitic acid(benzene-1,2,4,5-tetracarboxylic acid), mellitic acid (benzenehexacarboxylic acid), diphenic acid (biphenyl-2,2′-dicarboxylic acid),toluic acid (methylbenzoic acid), xylic acid, prehnitylic acid(2,3,4-trimethylbenzoic acid), γ-isodurylic acid (2,3,5-trimethylbenzoicacid), durylic acid (2,4,5-trimethylbenzoic acid), β-isodurylic acid(2,4,6-trimethylbenzoic acid), α-isodurylic acid (3,4,5-trimethylbenzoicacid), cuminic acid (4-isopropylbenzoic acid), uvitic acid(5-methylisophthalic acid), α-toluic acid (phenylacetic acid),hydratropic acid (2-phenylpropanoic acid), and hydrocinnamic acid(3-phenylpropanoic acid).

The rubber composition may include a salt of an aromatic carboxylic acidsubstituted with a hydroxyl group, an alkoxy group, or an oxo group.Examples of this carboxylic acid can include salicylic acid(2-hydroxybenzoic acid), anisic acid (methoxybenzoic acid), cresotinicacid (hydroxy(methyl) benzoicacid), o-homosalicylic acid(2-hydroxy-3-methylbenzoic acid), m-homosalicylic acid(2-hydroxy-4-methylbenzoic acid), p-homosalicylic acid(2-hydroxy-5-methylbenzoic acid), o-pyrocatechuic acid(2,3-dihydroxybenzoic acid), β-resorcylic acid (2,4-dihydroxybenzoicacid), γ-resorcylic acid (2,6-dihydroxybenzoic acid), protocatechuicacid (3,4-dihydroxybenzoic acid), α-resorcylic acid(3,5-dihydroxybenzoic acid), vanillic acid (4-hydroxy-3-methoxybenzoicacid), isovanillic acid (3-hydroxy-4-methoxybenzoic acid), veratric acid(3,4-dimethoxybenzoic acid), o-veratric acid (2,3-dimethoxybenzoicacid), orsellinic acid (2,4-dihydroxy-6-methylbenzoic acid), m-hemipinicacid (4,5-dimethoxyphthalic acid), gallic acid (3,4,5-trihydroxybenzoicacid), syringic acid (4-hydroxy-3,5-dimethoxybenzoic acid), asaronicacid (2,4,5-trimethoxybenzoic acid), mandelic acid(hydroxy(phenyl)acetic acid), vanillylmandelic acid(hydroxy(4-hydroxy-3-methoxyphenyl)acetic acid), homoanisic acid((4-methoxyphenyl)acetic acid), homogentisic acid((2,5-dihydroxyphenyl)acetic acid), homoprotocatechuic acid((3,4-dihydroxyphenyl)acetic acid), homovanillic acid((4-hydroxy-3-methoxyphenyl)acetic acid), homoisovanillic acid((3-hydroxy-4-methoxyphenyl)acetic acid), homoveratric acid((3,4-dimethoxyphenyl)acetic acid), o-homoveratric acid((2,3-dimethoxyphenyl)acetic acid), homophthalic acid(2-(carboxymethyl)benzoic acid), homoisophthalic acid(3-(carboxymethyl)benzoic acid), homoterephthalic acid(4-(carboxymethyl)benzoic acid), phthalonic acid(2-(carboxycarbonyl)benzoic acid), isophthalonic acid(3-(carboxycarbonyl)benzoic acid), terephthalonic acid(4-(carboxycarbonyl)benzoic acid), benzilic acid (hydroxydiphenylaceticacid), atrolactic acid (2-hydroxy-2-phenylpropanoic acid), tropic acid(3-hydroxy-2-phenylpropanoic acid), melilotic acid(3-(2-hydroxyphenyl)propanoic acid), phloretic acid(3-(4-hydroxyphenyl)propanoic acid), hydrocaffeic acid(3-(3,4-dihydroxyphenyl)propanoic acid), hydroferulic acid(3-(4-hydroxy-3-methoxyphenyl)propanoic acid), hydroisoferulic acid(3-(3-hydroxy-4-methoxyphenyl)propanoic acid), p-coumaric acid(3-(4-hydroxyphenyl)acrylic acid), umbellic acid(3-(2,4-dihydroxyphenyl)acrylic acid), caffeic acid(3-(3,4-dihydroxyphenyl)acrylic acid), ferulic acid(3-(4-hydroxy-3-methoxyphenyl)acrylic acid), isoferulic acid(3-(3-hydroxy-4-methoxyphenyl)acrylic acid), and sinapic acid(3-(4-hydroxy-3,5-dimethoxyphenyl)acrylic acid).

The cationic component of the carboxylate is a metal ion or an organiccation. Examples of the metal ion include sodium ion, potassium ion,lithium ion, silver ion, magnesium ion, calcium ion, zinc ion, bariumion, cadmium ion, copper ion, cobalt ion, nickel ion, manganese ion,aluminum ion, iron ion, tin ion, zirconium ion, and titanium ion. Two ormore types of ions may be used in combination.

The organic cation is a cation having a carbon chain. Examples of theorganic cation include organic ammonium ions. Examples of organicammonium ions include primary ammonium ions such as stearylammonium ion,hexylammonium ion, octylammonium ion, and 2-ethylhexylammonium ion;secondary ammonium ions such as dodecyl(lauryl)ammonium ion, andoctadecyl(stearyl)ammonium ion; tertiary ammonium ions such astrioctylammonium ion; and quaternary ammonium ions such asdioctyldimethylammonium ion, and distearyldimethylammonium ion. Two ormore types of organic cations may be used in combination.

Examples of preferable carboxylates include a potassium salt, amagnesium salt, an aluminum salt, a zinc salt, an iron salt, a coppersalt, a nickel salt, or a cobalt salt of myristic acid, palmitic acid,stearic acid, behenic acid, or oleic acid.

In light of linearity of the hardness distribution of the core 4, theamount of the carboxylate (d) is preferably equal to or greater than 1parts by weight, more preferably equal to or greater than 7.5 parts byweight, even more preferably equal to or greater than 10.0, andparticularly preferably equal to or greater than 12.0, per 100 parts byweight of the base rubber. In light of resilience performance, theamount is preferably equal to or less than 40 parts by weight, morepreferably equal to or less than 30 parts by weight, and particularlypreferably equal to or less than 20 parts by weight, per 100 parts byweight of the base rubber.

The weight ratio of the co-crosslinking agent (b) and the carboxylate(d) in the rubber composition is preferably equal to or greater than 3/7but equal to or less than 8/2, and is particularly preferably equal toor greater than 4/6 but equal to or less than 7/3. From the rubbercomposition in which this weight ratio is within the above range, thecore 4 whose hardness linearly increases from its central point towardits surface can be obtained.

As the co-crosslinking agent (b), zinc acrylate is preferably used. Zincacrylate whose surface is coated with a carboxylate for the purpose ofimproving dispersibility to rubber is present. When the rubbercomposition includes this zinc acrylate, the coating material serves asthe carboxylate (d). For example, when the rubber composition includes25 parts by weight of zinc acrylate that includes 10% by weight of acarboxylate, the amount of the carboxylate (d) is regarded as 2.5 partsby weight, and the amount of the zinc acrylate is regarded as 22.5 partsby weight.

The rubber composition preferably further includes an organic sulfurcompound (e). The organic sulfur compound (e) can contribute to controlof: the linearity of the hardness distribution of the core 4; and thedegree of the outer-hard/inner-soft structure. An example of the organicsulfur compound (e) is an organic compound having a thiol group or apolysulfide linkage having 2 to 4 sulfur atoms. A metal salt of thisorganic compound is also included in the organic sulfur compound (e).Examples of the organic sulfur compound (e) include aliphatic compoundssuch as aliphatic thiols, aliphatic thiocarboxylic acids, aliphaticdithiocarboxylic acids, and aliphatic polysulfides; heterocycliccompounds; alicyclic compounds such as alicyclic thiols, alicyclicthiocarboxylic acids, alicyclic dithiocarboxylic acids, and alicyclicpolysulfides; and aromatic compounds. Specific examples of the organicsulfur compound (e) include thiophenols, thionaphthols, polysulfides,thiocarboxylic acids, dithiocarboxylic acids, sulfenamides, thiurams,dithiocarbamates, and thiazoles. Preferable organic sulfur compounds arethiophenols, polysulfides having 2 to 4 sulfur atoms, thionaphthols,thiurams, and metal salts thereof.

Specific examples of the organic sulfur compound (e) are represented bythe following chemical formulas (1) to (4).

In the chemical formula (1), R1 to R5 each represent H or a substituent.

In the chemical formula (2), R1 to R10 each represent H or asubstituent.

In the chemical formula (3), R1 to R5 each represent H or a substituent,and M1 represents a monovalent metal atom.

In the chemical formula (4), R1 to R10 each represent H or asubstituent, and M2 represents a bivalent metal atom.

In the formulas (1) to (4), each substituent is at least one groupselected from the group consisting of a halogen group (F, Cl, Br, I), analkyl group, a carboxyl group (—COOH), an ester (—COOR) of a carboxylgroup, a formyl group (—CHO), an acyl group (—COR), a carbonyl halidegroup (—COX), a sulfo group (—SO₃H), an ester (—SO₃R) of a sulfo group,a sulfonyl halide group (—SO₂X), a sulfino group (—SO₂H), analkylsulfinyl group (—SOR), a carbamoyl group (—CONH₂), an alkyl halidegroup, a cyano group (—CN), and an alkoxy group (—OR).

Examples of the organic sulfur compound represented by the chemicalformula (1) include thiophenol; thiophenols substituted with halogengroups, such as 4-fluorothiophenol, 2,5-difluorothiophenol,2,4,5-trifluorothiophenol, 2,4,5,6-tetrafluorothiophenol,pentafluorothiophenol, 2-chlorothiophenol, 4-chlorothiophenol,2,4-dichlorothiophenol, 2,5-dichlorothiophenol,2,4,5-trichlorothiophenol, 2,4,5,6-tetrachlorothiophenol,pentachlorothiophenol, 4-bromothiophenol, 2,5-dibromothiophenol,2,4,5-tribromothiophenol, 2,4,5,6-tetrabromothiophenol,pentabromothiophenol, 4-iodothiophenol, 2,5-diiodothiophenol,2,4,5-triiodothiophenol, 2,4,5,6-tetraiodothiophenol, andpentaiodothiophenol; thiophenols substituted with alkyl groups, such as4-methylthiophenol, 2,4,5-trimethylthiophenol, pentamethylthiophenol,4-t-butylthiophenol, 2,4,5-tri-t-butylthiophenol, andpenta-t-butylthiophenol; thiophenols substituted with carboxyl groups,such as 4-carboxythiophenol, 2,4,6-tricarboxythiophenol, andpentacarboxythiophenol; thiophenols substituted with alkoxycarbonylgroups, such as 4-methoxycarbonylthiophenol,2,4,6-trimethoxycarbonylthiophenol, and pentamethoxycarbonylthiophenol;thiophenols substituted with formyl groups, such as 4-formylthiophenol,2,4,6-triformylthiophenol, and pentaformylthiophenol; thiophenolssubstituted with acyl groups, such as 4-acetylthiophenol,2,4,6-triacetylthiophenol, and pentaacetylthiophenol; thiophenolssubstituted with carbonyl halide groups, such as4-chlorocarbonylthiophenol, 2,4,6-tri(chlorocarbonyl)thiophenol, andpenta(chlorocarbonyl)thiophenol; thiophenols substituted with sulfogroups, such as 4-sulfothiophenol, 2,4,6-trisulfothiophenol, andpentasulfothiophenol; thiophenols substituted with alkoxysulfonylgroups, such as 4-methoxysulfonylthiophenol,2,4,6-trimethoxysulfonylthiophenol, and pentamethoxysulfonylthiophenol;thiophenols substituted with sulfonyl halide groups, such as4-chlorosulfonylthiophenol, 2,4,6-tri(chlorosulfonyl)thiophenol, andpenta(chlorosulfonyl)thiophenol; thiophenols substituted with sulfinogroups, such as 4-sulfinothiophenol, 2,4,6-trisulfinothiophenol, andpentasulfinothiophenol; thiophenols substituted with alkylsulfinylgroups, such as 4-methylsulfinylthiophenol,2,4,6-tri(methylsulfinyl)thiophenol, andpenta(methylsulfinyl)thiophenol; thiophenols substituted with carbamoylgroups, such as 4-carbamoylthiophenol, 2,4,6-tricarbamoylthiophenol, andpentacarbamoylthiophenol; thiophenols substituted with alkyl halidegroups, such as 4-trichloromethylthiophenol,2,4,6-tri(trichloromethyl)thiophenol, andpenta(trichloromethyl)thiophenol; thiophenols substituted with cyanogroups, such as 4-cyanothiophenol, 2,4,6-tricyanothiophenol, andpentacyanothiophenol; and thiophenols substituted with alkoxy groups,such as 4-methoxythiophenol, 2,4,6-trimethoxythiophenol, andpentamethoxythiophenol. Each of these thiophenols is substituted withone type of substituent.

Another example of the organic sulfur compound represented by thechemical formula (1) is a compound substituted with at least one type ofthe above substituents and another substituent. Examples of the othersubstituent include a nitro group (—NO₂), an amino group (—NH₂), ahydroxyl group (—OH), and a phenylthio group (—SPh). Specific examplesof the compound include 4-chloro-2-nitrothiophenol,4-chloro-2-aminothiophenol, 4-chloro-2-hydroxythiophenol,4-chloro-2-phenylthiothiophenol, 4-methyl-2-nitrothiophenol,4-methyl-2-aminothiophenol, 4-methyl-2-hydroxythiophenol,4-methyl-2-phenylthiothiophenol, 4-carboxy-2-nitrothiophenol,4-carboxy-2-aminothiophenol, 4-carboxy-2-hydroxythiophenol,4-carboxy-2-phenylthiothiophenol, 4-methoxycarbonyl-2-nitrothiophenol,4-methoxycarbonyl-2-aminothiophenol,4-methoxycarbonyl-2-hydroxythiophenol,4-methoxycarbonyl-2-phenylthiothiophenol, 4-formyl-2-nitrothiophenol,4-formyl-2-aminothiophenol, 4-formyl-2-hydroxythiophenol,4-formyl-2-phenylthiothiophenol, 4-acetyl-2-nitrothiophenol,4-acetyl-2-aminothiophenol, 4-acetyl-2-hydroxythiophenol,4-acetyl-2-phenylthiothiophenol, 4-chlorocarbonyl-2-nitrothiophenol,4-chlorocarbonyl-2-aminothiophenol,4-chlorocarbonyl-2-hydroxythiophenol,4-chlorocarbonyl-2-phenylthiothiophenol, 4-sulfo-2-nitrothiophenol,4-sulfo-2-aminothiophenol, 4-sulfo-2-hydroxythiophenol,4-sulfo-2-phenylthiothiophenol, 4-methoxysulfonyl-2-nitrothiophenol,4-methoxysulfonyl-2-aminothiophenol,4-methoxysulfonyl-2-hydroxythiophenol,4-methoxysulfonyl-2-phenylthiothiophenol,4-chlorosulfonyl-2-nitrothiophenol, 4-chlorosulfonyl-2-aminothiophenol,4-chlorosulfonyl-2-hydroxythiophenol,4-chlorosulfonyl-2-phenylthiothiophenol, 4-sulfino-2-nitrothiophenol,4-sulfino-2-aminothiophenol, 4-sulfino-2-hydroxythiophenol,4-sulfino-2-phenylthiothiophenol, 4-methylsulfinyl-2-nitrothiophenol,4-methyl-2-aminosulfinylthiophenol,4-methylsulfinyl-2-hydroxythiophenol,4-methylsulfinyl-2-phenylthiothiophenol, 4-carbamoyl-2-nitrothiophenol,4-carbamoyl-2-aminothiophenol, 4-carbamoyl-2-hydroxythiophenol,4-carbamoyl-2-phenylthiothiophenol, 4-trichloromethyl-2-nitrothiophenol,4-trichloromethyl-2-aminothiophenol,4-trichloromethyl-2-hydroxythiophenol,4-trichloromethyl-2-phenylthiothiophenol, 4-cyano-2-nitrothiophenol,4-cyano-2-aminothiophenol, 4-cyano-2-hydroxythiophenol,4-cyano-2-phenylthiothiophenol, 4-methoxy-2-nitrothiophenol,4-methoxy-2-aminothiophenol, 4-methoxy-2-hydroxythiophenol, and4-methoxy-2-phenylthiothiophenol.

Still another example of the organic sulfur compound represented by thechemical formula (1) is a compound substituted with two or more types ofsubstituents. Specific examples of the compound include4-acetyl-2-chlorothiophenol, 4-acetyl-2-methylthiophenol,4-acetyl-2-carboxythiophenol, 4-acetyl-2-methoxycarbonylthiophenol,4-acetyl-2-formylthiophenol, 4-acetyl-2-chlorocarbonylthiophenol,4-acetyl-2-sulfothiophenol, 4-acetyl-2-methoxysulfonylthiophenol,4-acetyl-2-chlorosulfonylthiophenol, 4-acetyl-2-sulfinothiophenol,4-acetyl-2-methylsulfinylthiophenol, 4-acetyl-2-carbamoylthiophenol,4-acetyl-2-trichloromethylthiophenol, 4-acetyl-2-cyanothiophenol, and4-acetyl-2-methoxythiophenol.

Examples of the organic sulfur compound represented by the chemicalformula (2) include diphenyl disulfide; diphenyl disulfides substitutedwith halogen groups, such as bis(4-fluorophenyl)disulfide,bis(2,5-difluorophenyl)disulfide, bis(2,4,5-trifluorophenyl)disulfide,bis(2,4,5,6-tetrafluorophenyl)disulfide,bis(pentafluorophenyl)disulfide, bis(4-chlorophenyl)disulfide,bis(2,5-dichlorophenyl)disulfide, bis(2,4,5-trichlorophenyl)disulfide,bis(2,4,5,6-tetrachlorophenyl)disulfide,bis(pentachlorophenyl)disulfide, bis(4-bromophenyl)disulfide,bis(2,5-dibromophenyl)disulfide, bis(2,4,5-tribromophenyl)disulfide,bis(2,4,5,6-tetrabromophenyl)disulfide, bis(pentabromophenyl)disulfide,bis(4-iodophenyl)disulfide, bis(2,5-diiodophenyl)disulfide,bis(2,4,5-triiodophenyl)disulfide,bis(2,4,5,6-tetraiodophenyl)disulfide, andbis(pentaiodophenyl)disulfide; diphenyl disulfides substituted withalkyl groups, such as bis(4-methylphenyl)disulfide,bis(2,4,5-trimethylphenyl)disulfide, bis(pentamethylphenyl)disulfide,bis(4-t-butylphenyl)disulfide, bis(2,4,5-tri-t-butylphenyl)disulfide,and bis(penta-t-butylphenyl)disulfide; diphenyl disulfides substitutedwith carboxyl groups, such as bis(4-carboxyphenyl)disulfide,bis(2,4,6-tricarboxyphenyl)disulfide, andbis(pentacarboxyphenyl)disulfide; diphenyl disulfides substituted withalkoxycarbonyl groups, such as bis(4-methoxycarbonylphenyl)disulfide,bis(2,4,6-trimethoxycarbonylphenyl)disulfide, andbis(pentamethoxycarbonylphenyl)disulfide; diphenyl disulfidessubstituted with formyl groups, such as bis(4-formylphenyl)disulfide,bis(2,4,6-triformylphenyl)disulfide, andbis(pentaformylphenyl)disulfide; diphenyl disulfides substituted withacyl groups, such as bis(4-acetylphenyl)disulfide,bis(2,4,6-triacetylphenyl)disulfide, andbis(pentaacetylphenyl)disulfide; diphenyl disulfides substituted withcarbonyl halide groups, such as bis(4-chlorocarbonylphenyl)disulfide,bis(2,4,6-tri(chlorocarbonyl)phenyl)disulfide, andbis(penta(chlorocarbonyl)phenyl)disulfide; diphenyl disulfidessubstituted with sulfo groups, such as bis(4-sulfophenyl)disulfide,bis(2,4,6-trisulfophenyl)disulfide, and bis(pentasulfophenyl)disulfide;diphenyl disulfides substituted with alkoxysulfonyl groups, such asbis(4-methoxysulfonylphenyl)disulfide,bis(2,4,6-trimethoxysulfonylphenyl)disulfide, andbis(pentamethoxysulfonylphenyl)disulfide; diphenyl disulfidessubstituted with sulfonyl halide groups, such asbis(4-chlorosulfonylphenyl)disulfide,bis(2,4,6-tri(chlorosulfonyl)phenyl)disulfide, andbis(penta(chlorosulfonyl)phenyl)disulfide; diphenyl disulfidessubstituted with sulfino groups, such as bis(4-sulfinophenyl)disulfide,bis(2,4,6-trisulfinophenyl)disulfide, andbis(pentasulfinophenyl)disulfide; diphenyl disulfides substituted withalkylsulfinyl groups, such as bis(4-methylsulfinylphenyl)disulfide,bis(2,4,6-tri(methylsulfinyl)phenyl)disulfide, andbis(penta(methylsulfinyl)phenyl)disulfide; diphenyl disulfidessubstituted with carbamoyl groups, such asbis(4-carbamoylphenyl)disulfide, bis(2,4,6-tricarbamoylphenyl)disulfide,and bis(pentacarbamoylphenyl)disulfide; diphenyl disulfides substitutedwith alkyl halide groups, such as bis(4-trichloromethylphenyl)disulfide,bis(2,4,6-tri(trichloromethyl)phenyl)disulfide, andbis(penta(trichloromethyl)phenyl)disulfide; diphenyl disulfidessubstituted with cyano groups, such as bis(4-cyanophenyl)disulfide,bis(2,4,6-tricyanophenyl)disulfide, and bis(pentacyanophenyl)disulfide;and diphenyl disulfides substituted with alkoxy groups, such asbis(4-methoxyphenyl)disulfide, bis(2,4,6-trimethoxyphenyl)disulfide, andbis(pentamethoxyphenyl)disulfide. Each of these diphenyl disulfides issubstituted with one type of substituent.

Another example of the organic sulfur compound represented by thechemical formula (2) is a compound substituted with at least one type ofthe above substituents and another substituent. Examples of the othersubstituent include a nitro group (—NO₂), an amino group (—NH₂), ahydroxyl group (—OH), and a phenylthio group (—SPh). Specific examplesof the compound include bis(4-chloro-2-nitrophenyl)disulfide,bis(4-chloro-2-aminophenyl)disulfide,bis(4-chloro-2-hydroxyphenyl)disulfide,bis(4-chloro-2-phenylthiophenyl)disulfide,bis(4-methyl-2-nitrophenyl)disulfide,bis(4-methyl-2-aminophenyl)disulfide,bis(4-methyl-2-hydroxyphenyl)disulfide,bis(4-methyl-2-phenylthiophenyl)disulfide,bis(4-carboxy-2-nitrophenyl)disulfide,bis(4-carboxy-2-aminophenyl)disulfide,bis(4-carboxy-2-hydroxyphenyl)disulfide,bis(4-carboxy-2-phenylthiophenyl)disulfide,bis(4-methoxycarbonyl-2-nitrophenyl)disulfide,bis(4-methoxycarbonyl-2-aminophenyl)disulfide,bis(4-methoxycarbonyl-2-hydroxyphenyl)disulfide,bis(4-methoxycarbonyl-2-phenylthiophenyl)disulfide,bis(4-formyl-2-nitrophenyl)disulfide,bis(4-formyl-2-aminophenyl)disulfide,bis(4-formyl-2-hydroxyphenyl)disulfide,bis(4-formyl-2-phenylthiophenyl)disulfide,bis(4-acetyl-2-nitrophenyl)disulfide,bis(4-acetyl-2-aminophenyl)disulfide,bis(4-acetyl-2-hydroxyphenyl)disulfide,bis(4-acetyl-2-phenylthiophenyl)disulfide,bis(4-chlorocarbonyl-2-nitrophenyl)disulfide,bis(4-chlorocarbonyl-2-aminophenyl)disulfide,bis(4-chlorocarbonyl-2-hydroxyphenyl)disulfide,bis(4-chlorocarbonyl-2-phenylthiophenyl)disulfide,bis(4-sulfo-2-nitrophenyl)disulfide,bis(4-sulfo-2-aminophenyl)disulfide,bis(4-sulfo-2-hydroxyphenyl)disulfide,bis(4-sulfo-2-phenylthiophenyl)disulfide,bis(4-methoxysulfonyl-2-nitrophenyl)disulfide,bis(4-methoxysulfonyl-2-aminophenyl)disulfide,bis(4-methoxysulfonyl-2-hydroxyphenyl)disulfide,bis(4-methoxysulfonyl-2-phenylthiophenyl)disulfide,bis(4-chlorosulfonyl-2-nitrophenyl)disulfide,bis(4-chlorosulfonyl-2-aminophenyl)disulfide,bis(4-chlorosulfonyl-2-hydroxyphenyl)disulfide,bis(4-chlorosulfonyl-2-phenylthiophenyl)disulfide,bis(4-sulfino-2-nitrophenyl)disulfide,bis(4-sulfino-2-aminophenyl)disulfide,bis(4-sulfino-2-hydroxyphenyl)disulfide,bis(4-sulfino-2-phenylthiophenyl)disulfide,bis(4-methylsulfinyl-2-nitrophenyl)disulfide,bis(4-methylsulfinyl-2-aminophenyl)disulfide,bis(4-methylsulfinyl-2-hydroxyphenyl)disulfide,bis(4-methylsulfinyl-2-phenylthiophenyl)disulfide,bis(4-carbamoyl-2-nitrophenyl)disulfide,bis(4-carbamoyl-2-aminophenyl)disulfide,bis(4-carbamoyl-2-hydroxyphenyl)disulfide,bis(4-carbamoyl-2-phenylthiophenyl)disulfide,bis(4-trichloromethyl-2-nitrophenyl)disulfide,bis(4-trichloromethyl-2-aminophenyl)disulfide,bis(4-trichloromethyl-2-hydroxyphenyl)disulfide,bis(4-trichloromethyl-2-phenylthiophenyl)disulfide,bis(4-cyano-2-nitrophenyl)disulfide,bis(4-cyano-2-aminophenyl)disulfide,bis(4-cyano-2-hydroxyphenyl)disulfide,bis(4-cyano-2-phenylthiophenyl)disulfide,bis(4-methoxy-2-nitrophenyl)disulfide,bis(4-methoxy-2-aminophenyl)disulfide,bis(4-methoxy-2-hydroxyphenyl)disulfide, andbis(4-methoxy-2-phenylthiophenyl)disulfide.

Sill another example of the organic sulfur compound represented by thechemical formula (2) is a compound substituted with two or more types ofsubstituents. Specific examples of the compound includebis(4-acetyl-2-chlorophenyl)disulfide,bis(4-acetyl-2-methylphenyl)disulfide,bis(4-acetyl-2-carboxyphenyl)disulfide,bis(4-acetyl-2-methoxycarbonylphenyl)disulfide,bis(4-acetyl-2-formylphenyl)disulfide,bis(4-acetyl-2-chlorocarbonylphenyl)disulfide,bis(4-acetyl-2-sulfophenyl)disulfide,bis(4-acetyl-2-methoxysulfonylphenyl)disulfide,bis(4-acetyl-2-chlorosulfonylphenyl)disulfide,bis(4-acetyl-2-sulfinophenyl)disulfide,bis(4-acetyl-2-methylsulfinylphenyl)disulfide,bis(4-acetyl-2-carbamoylphenyl)disulfide,bis(4-acetyl-2-trichloromethylphenyl)disulfide,bis(4-acetyl-2-cyanophenyl)disulfide, andbis(4-acetyl-2-methoxyphenyl)disulfide.

Examples of the organic sulfur compound represented by the chemicalformula (3) include thiophenol sodium salt; thiophenol sodium saltssubstituted with halogen groups, such as 4-fluorothiophenol sodium salt,2,5-difluorothiophenol sodium salt, 2,4,5-trifluorothiophenol sodiumsalt, 2,4,5,6-tetrafluorothiophenol sodium salt, pentafluorothiophenolsodium salt, 4-chlorothiophenol sodium salt, 2,5-dichlorothiophenolsodium salt, 2,4,5-trichlorothiophenol sodium salt,2,4,5,6-tetrachlorothiophenol sodium salt, pentachlorothiophenol sodiumsalt, 4-bromothiophenol sodium salt, 2,5-dibromothiophenol sodium salt,2,4,5-tribromothiophenol sodium salt, 2,4,5,6-tetrabromothiophenolsodium salt, pentabromothiophenol sodium salt, 4-iodothiophenol sodiumsalt, 2,5-diiodothiophenol sodium salt, 2,4,5-triiodothiophenol sodiumsalt, 2,4,5,6-tetraiodothiophenol sodium salt, and pentaiodothiophenolsodium salt; thiophenol sodium salts substituted with alkyl groups, suchas 4-methylthiophenol sodium salt, 2,4,5-trimethylthiophenol sodiumsalt, pentamethylthiophenol sodium salt, 4-t-butylthiophenol sodiumsalt, 2,4,5-tri-t-butylthiophenol sodium salt, andpenta(t-butyl)thiophenol sodium salt; thiophenol sodium saltssubstituted with carboxyl groups, such as 4-carboxythiophenol sodiumsalt, 2,4,6-tricarboxythiophenol sodium salt, and pentacarboxythiophenolsodium salt; thiophenol sodium salts substituted with alkoxycarbonylgroups, such as 4-methoxycarbonylthiophenol sodium salt,2,4,6-trimethoxycarbonylthiophenol sodium salt, andpentamethoxycarbonylthiophenol sodium salt; thiophenol sodium saltssubstituted with formyl groups, such as 4-formylthiophenol sodium salt,2,4,6-triformylthiophenol sodium salt, and pentaformylthiophenol sodiumsalt; thiophenol sodium salts substituted with acyl groups, such as4-acetylthiophenol sodium salt, 2,4,6-triacetylthiophenol sodium salt,and pentaacetylthiophenol sodium salt; thiophenol sodium saltssubstituted with carbonyl halide groups, such as4-chlorocarbonylthiophenol sodium salt,2,4,6-tri(chlorocarbonyl)thiophenol sodium salt, andpenta(chlorocarbonyl)thiophenol sodium salt; thiophenol sodium saltssubstituted with sulfo groups, such as 4-sulfothiophenol sodium salt,2,4,6-trisulfothiophenol sodium salt, and pentasulfothiophenol sodiumsalt; thiophenol sodium salts substituted with alkoxysulfonyl groups,such as 4-methoxysulfonylthiophenol sodium salt,2,4,6-trimethoxysulfonylthiophenol sodium salt, andpentamethoxysulfonylthiophenol sodium salt; thiophenol sodium saltssubstituted with sulfonyl halide groups, such as4-chlorosulfonylthiophenol sodium salt,2,4,6-tri(chlorosulfonyl)thiophenol sodium salt, andpenta(chlorosulfonyl)thiophenol sodium salt; thiophenol sodium saltssubstituted with sulfino groups, such as 4-sulfinothiophenol sodiumsalt, 2,4,6-trisulfinothiophenol sodium salt, and pentasulfinothiophenolsodium salt; thiophenol sodium salts substituted with alkylsulfinylgroups, such as 4-methylsulfinylthiophenol sodium salt,2,4,6-tri(methylsulfinyl)thiophenol sodium salt, andpenta(methylsulfinyl)thiophenol sodium salt; thiophenol sodium saltssubstituted with carbamoyl groups, such as 4-carbamoylthiophenol sodiumsalt, 2,4,6-tricarbamoylthiophenol sodium salt, andpentacarbamoylthiophenol sodium salt; thiophenol sodium saltssubstituted with alkyl halide groups, such as4-trichloromethylthiophenol sodium salt,2,4,6-tri(trichloromethyl)thiophenol sodium salt, andpenta(trichloromethyl)thiophenol sodium salt; thiophenol sodium saltssubstituted with cyano groups, such as 4-cyanothiophenol sodium salt,2,4,6-tricyanothiophenol sodium salt, and pentacyanothiophenol sodiumsalt; and thiophenol sodium salts substituted with alkoxy groups, suchas 4-methoxythiophenol sodium salt, 2,4,6-trimethoxythiophenol sodiumsalt, and pentamethoxythiophenol sodium salt. Each of these thiophenolsodium salts is substituted with one type of substituent.

Another example of the organic sulfur compound represented by thechemical formula (3) is a compound substituted with at least one type ofthe above substituents and another substituent. Examples of the othersubstituent include a nitro group (—NO₂), an amino group (—NH₂), ahydroxyl group (—OH), and a phenylthio group (—SPh). Specific examplesof the compound include 4-chloro-2-nitrothiophenol sodium salt,4-chloro-2-aminothiophenol sodium salt, 4-chloro-2-hydroxythiophenolsodium salt, 4-chloro-2-phenylthiothiophenol sodium salt,4-methyl-2-nitrothiophenol sodium salt, 4-methyl-2-aminothiophenolsodium salt, 4-methyl-2-hydroxythiophenol sodium salt,4-methyl-2-phenylthiothiophenol sodium salt, 4-carboxy-2-nitrothiophenolsodium salt, 4-carboxy-2-aminothiophenol sodium salt,4-carboxy-2-hydroxythiophenol sodium salt,4-carboxy-2-phenylthiothiophenol sodium salt,4-methoxycarbonyl-2-nitrothiophenol sodium salt,4-methoxycarbonyl-2-aminothiophenol sodium salt,4-methoxycarbonyl-2-hydroxythiophenol sodium salt,4-methoxycarbonyl-2-phenylthiothiophenol sodium salt,4-formyl-2-nitrothiophenol sodium salt, 4-formyl-2-aminothiophenolsodium salt, 4-formyl-2-hydroxythiophenol sodium salt,4-formyl-2-phenylthiothiophenol sodium salt, 4-acetyl-2-nitrothiophenolsodium salt, 4-acetyl-2-aminothiophenol sodium salt,4-acetyl-2-hydroxythiophenol sodium salt,4-acetyl-2-phenylthiothiophenol sodium salt,4-chlorocarbonyl-2-nitrothiophenol sodium salt,4-chlorocarbonyl-2-aminothiophenol sodium salt,4-chlorocarbonyl-2-hydroxythiophenol sodium salt,4-chlorocarbonyl-2-phenylthiothiophenol sodium salt,4-sulfo-2-nitrothiophenol sodium salt, 4-sulfo-2-aminothiophenol sodiumsalt, 4-sulfo-2-hydroxythiophenol sodium salt,4-sulfo-2-phenylthiothiophenol sodium salt,4-methoxysulfonyl-2-nitrothiophenol sodium salt,4-methoxysulfonyl-2-aminothiophenol sodium salt,4-methoxysulfonyl-2-hydroxythiophenol sodium salt,4-methoxysulfonyl-2-phenylthiothiophenol sodium salt,4-chlorosulfonyl-2-nitrothiophenol sodium salt,4-chlorosulfonyl-2-aminothiophenol sodium salt,4-chlorosulfonyl-2-hydroxythiophenol sodium salt,4-chlorosulfonyl-2-phenylthiothiophenol sodium salt,4-sulfino-2-nitrothiophenol sodium salt, 4-sulfino-2-aminothiophenolsodium salt, 4-sulfino-2-hydroxythiophenol sodium salt,4-sulfino-2-phenylthiothiophenol sodium salt,4-methylsulfinyl-2-nitrothiophenol sodium salt,4-methylsulfinyl-2-aminothiophenol sodium salt,4-methylsulfinyl-2-hydroxythiophenol sodium salt,4-methylsulfinyl-2-phenylthiothiophenol sodium salt,4-carbamoyl-2-nitrothiophenol sodium salt, 4-carbamoyl-2-aminothiophenolsodium salt, 4-carbamoyl-2-hydroxythiophenol sodium salt,4-carbamoyl-2-phenylthiothiophenol sodium salt,4-trichloromethyl-2-nitrothiophenol sodium salt,4-trichloromethyl-2-aminothiophenol sodium salt,4-trichloromethyl-2-hydroxythiophenol sodium salt,4-trichloromethyl-2-phenylthiothiophenol sodium salt,4-cyano-2-nitrothiophenol sodium salt, 4-cyano-2-aminothiophenol sodiumsalt, 4-cyano-2-hydroxythiophenol sodium salt,4-cyano-2-phenylthiothiophenol sodium salt, 4-methoxy-2-nitrothiophenolsodium salt, 4-methoxy-2-aminothiophenol sodium salt,4-methoxy-2-hydroxythiophenol sodium salt, and4-methoxy-2-phenylthiothiophenol sodium salt.

Still another example of the organic sulfur compound represented by thechemical formula (3) is a compound substituted with two or more types ofsubstituents. Specific examples of the compound include4-acetyl-2-chlorothiophenol sodium salt, 4-acetyl-2-methylthiophenolsodium salt, 4-acetyl-2-carboxythiophenol sodium salt,4-acetyl-2-methoxycarbonylthiophenol sodium salt,4-acetyl-2-formylthiophenol sodium salt,4-acetyl-2-chlorocarbonylthiophenol sodium salt,4-acetyl-2-sulfothiophenol sodium salt,4-acetyl-2-methoxysulfonylthiophenol sodium salt,4-acetyl-2-chlorosulfonylthiophenol sodium salt,4-acetyl-2-sulfinothiophenol sodium salt,4-acetyl-2-methylsulfinylthiophenol sodium salt,4-acetyl-2-carbamoylthiophenol sodium salt,4-acetyl-2-trichloromethylthiophenol sodium salt,4-acetyl-2-cyanothiophenol sodium salt, and 4-acetyl-2-methoxythiophenolsodium salt. Examples of the monovalent metal represented by M1 in thechemical formula (3) include sodium, lithium, potassium, copper (I), andsilver (I).

Examples of the organic sulfur compound represented by the chemicalformula (4) include thiophenol zinc salt; thiophenol zinc saltssubstituted with halogen groups, such as 4-fluorothiophenol zinc salt,2,5-difluorothiophenol zinc salt, 2,4,5-trifluorothiophenol zinc salt,2,4,5,6-tetrafluorothiophenol zinc salt, pentafluorothiophenol zincsalt, 4-chlorothiophenol zinc salt, 2,5-dichlorothiophenol zinc salt,2,4,5-trichlorothiophenol zinc salt, 2,4,5,6-tetrachlorothiophenol zincsalt, pentachlorothiophenol zinc salt, 4-bromothiophenol zinc salt,2,5-dibromothiophenol zinc salt, 2,4,5-tribromothiophenol zinc salt,2,4,5,6-tetrabromothiophenol zinc salt, pentabromothiophenol zinc salt,4-iodothiophenol zinc salt, 2,5-diiodothiophenol zinc salt,2,4,5-triiodothiophenol zinc salt, 2,4,5,6-tetraiodothiophenol zincsalt, and pentaiodothiophenol zinc salt; thiophenol zinc saltssubstituted with alkyl groups, such as 4-methylthiophenol zinc salt,2,4,5-trimethylthiophenol zinc salt, pentamethylthiophenol zinc salt,4-t-butylthiophenol zinc salt, 2,4,5-tri-t-butylthiophenol zinc salt,and penta-t-butylthiophenol zinc salt; thiophenol zinc salts substitutedwith carboxyl groups, such as 4-carboxythiophenol zinc salt,2,4,6-tricarboxythiophenol zinc salt, and pentacarboxythiophenol zincsalt; thiophenol zinc salts substituted with alkoxycarbonyl groups, suchas 4-methoxycarbonylthiophenol zinc salt,2,4,6-trimethoxycarbonylthiophenol zinc salt, andpentamethoxycarbonylthiophenol zinc salt; thiophenol zinc saltssubstituted with formyl groups, such as 4-formylthiophenol zinc salt,2,4,6-triformylthiophenol zinc salt, and pentaformylthiophenol zincsalt; thiophenol zinc salts substituted with acyl groups, such as4-acetylthiophenol zinc salt, 2,4,6-triacetylthiophenol zinc salt, andpentaacetylthiophenol zinc salt; thiophenol zinc salts substituted withcarbonyl halide groups, such as 4-chlorocarbonylthiophenol zinc salt,2,4,6-tri(chlorocarbonyl)thiophenol zinc salt, andpenta(chlorocarbonyl)thiophenol zinc salt; thiophenol zinc saltssubstituted with sulfo groups, such as 4-sulfothiophenol zinc salt,2,4,6-trisulfothiophenol zinc salt, and pentasulfothiophenol zinc salt;thiophenol zinc salts substituted with alkoxysulfonyl groups, such as4-methoxysulfonylthiophenol zinc salt,2,4,6-trimethoxysulfonylthiophenol zinc salt, andpentamethoxysulfonylthiophenol zinc salt; thiophenol zinc saltssubstituted with sulfonyl halide groups, such as4-chlorosulfonylthiophenol zinc salt,2,4,6-tri(chlorosulfonyl)thiophenol zinc salt, andpenta(chlorosulfonyl)thiophenol zinc salt; thiophenol zinc saltssubstituted with sulfino groups, such as 4-sulfinothiophenol zinc salt,2,4,6-trisulfinothiophenol zinc salt, and pentasulfinothiophenol zincsalt; thiophenol zinc salts substituted with alkylsulfinyl groups, suchas 4-methylsulfinylthiophenol zinc salt,2,4,6-tri(methylsulfinyl)thiophenol zinc salt, andpenta(methylsulfinyl)thiophenol zinc salt; thiophenol zinc saltssubstituted with carbamoyl groups, such as 4-carbamoylthiophenol zincsalt, 2,4,6-tricarbamoylthiophenol zinc salt, andpentacarbamoylthiophenol zinc salt; thiophenol zinc salts substitutedwith alkyl halide groups, such as 4-trichloromethylthiophenol zinc salt,2,4,6-tri(trichloromethyl)thiophenol zinc salt, andpenta(trichloromethyl)thiophenol zinc salt; thiophenol zinc saltssubstituted with cyano groups, such as 4-cyanothiophenol zinc salt,2,4,6-tricyanothiophenol zinc salt, and pentacyanothiophenol zinc salt;and thiophenol zinc salts substituted with alkoxy groups, such as4-methoxythiophenol zinc salt, 2,4,6-trimethoxythiophenol zinc salt, andpentamethoxythiophenol zinc salt. Each of these thiophenol zinc salts issubstituted with one type of substituent.

Another example of the organic sulfur compound represented by thechemical formula (4) is a compound substituted with at least one type ofthe above substituents and another substituent. Examples of the othersubstituent include a nitro group (—NO₂), an amino group (—NH₂), ahydroxyl group (—OH), and a phenylthio group (—SPh). Specific examplesof the compound include 4-chloro-2-nitrothiophenol zinc salt,4-chloro-2-aminothiophenol zinc salt, 4-chloro-2-hydroxythiophenol zincsalt, 4-chloro-2-phenylthiothiophenol zinc salt,4-methyl-2-nitrothiophenol zinc salt, 4-methyl-2-aminothiophenol zincsalt, 4-methyl-2-hydroxythiophenol zinc salt,4-methyl-2-phenylthiothiophenol zinc salt, 4-carboxy-2-nitrothiophenolzinc salt, 4-carboxy-2-aminothiophenol zinc salt,4-carboxy-2-hydroxythiophenol zinc salt,4-carboxy-2-phenylthiothiophenol zinc salt,4-methoxycarbonyl-2-nitrothiophenol zinc salt,4-methoxycarbonyl-2-aminothiophenol zinc salt,4-methoxycarbonyl-2-hydroxythiophenol zinc salt,4-methoxycarbonyl-2-phenylthiothiophenol zinc salt,4-formyl-2-nitrothiophenol zinc salt, 4-formyl-2-aminothiophenol zincsalt, 4-formyl-2-hydroxythiophenol zinc salt,4-formyl-2-phenylthiothiophenol zinc salt, 4-acetyl-2-nitrothiophenolzinc salt, 4-acetyl-2-aminothiophenol zinc salt,4-acetyl-2-hydroxythiophenol zinc salt, 4-acetyl-2-phenylthiothiophenolzinc salt, 4-chlorocarbonyl-2-nitrothiophenol zinc salt,4-chlorocarbonyl-2-aminothiophenol zinc salt,4-chlorocarbonyl-2-hydroxythiophenol zinc salt,4-chlorocarbonyl-2-phenylthiothiophenol zinc salt,4-sulfo-2-nitrothiophenol zinc salt, 4-sulfo-2-aminothiophenol zincsalt, 4-sulfo-2-hydroxythiophenol zinc salt,4-sulfo-2-phenylthiothiophenol zinc salt,4-methoxysulfonyl-2-nitrothiophenol zinc salt,4-methoxysulfonyl-2-aminothiophenol zinc salt,4-methoxysulfonyl-2-hydroxythiophenol zinc salt,4-methoxysulfonyl-2-phenylthiothiophenol zinc salt,4-chlorosulfonyl-2-nitrothiophenol zinc salt,4-chlorosulfonyl-2-aminothiophenol zinc salt,4-chlorosulfonyl-2-hydroxythiophenol zinc salt,4-chlorosulfonyl-2-phenylthiothiophenol zinc salt,4-sulfino-2-nitrothiophenol zinc salt, 4-sulfino-2-aminothiophenol zincsalt, 4-sulfino-2-hydroxythiophenol zinc salt,4-sulfino-2-phenylthiothiophenol zinc salt,4-methylsulfinyl-2-nitrothiophenol zinc salt,4-methylsulfinyl-2-aminothiophenol zinc salt,4-methylsulfinyl-2-hydroxythiophenol zinc salt,4-methylsulfinyl-2-phenylthiothiophenol zinc salt,4-carbamoyl-2-nitrothiophenol zinc salt, 4-carbamoyl-2-aminothiophenolzinc salt, 4-carbamoyl-2-hydroxythiophenol zinc salt,4-carbamoyl-2-phenylthiothiophenol zinc salt,4-trichloromethyl-2-nitrothiophenol zinc salt,4-trichloromethyl-2-aminothiophenol zinc salt,4-trichloromethyl-2-hydroxythiophenol zinc salt,4-trichloromethyl-2-phenylthiothiophenol zinc salt,4-cyano-2-nitrothiophenol zinc salt, 4-cyano-2-aminothiophenol zincsalt, 4-cyano-2-hydroxythiophenol zinc salt,4-cyano-2-phenylthiothiophenol zinc salt, 4-methoxy-2-nitrothiophenolzinc salt, 4-methoxy-2-aminothiophenol zinc salt,4-methoxy-2-hydroxythiophenol zinc salt, and4-methoxy-2-phenylthiothiophenol zinc salt.

Still another example of the organic sulfur compound represented by thechemical formula (4) is a compound substituted with two or more types ofsubstituents. Specific examples of the compound include4-acetyl-2-chlorothiophenol zinc salt, 4-acetyl-2-methylthiophenol zincsalt, 4-acetyl-2-carboxythiophenol zinc salt,4-acetyl-2-methoxycarbonylthiophenol zinc salt,4-acetyl-2-formylthiophenol zinc salt,4-acetyl-2-chlorocarbonylthiophenol zinc salt,4-acetyl-2-sulfothiophenol zinc salt,4-acetyl-2-methoxysulfonylthiophenol zinc salt,4-acetyl-2-chlorosulfonylthiophenol zinc salt,4-acetyl-2-sulfinothiophenol zinc salt,4-acetyl-2-methylsulfinylthiophenol zinc salt,4-acetyl-2-carbamoylthiophenol zinc salt,4-acetyl-2-trichloromethylthiophenol zinc salt,4-acetyl-2-cyanothiophenol zinc salt, and 4-acetyl-2-methoxythiophenolzinc salt. Examples of the bivalent metal represented by M2 in thechemical formula (4) include zinc, magnesium, calcium, strontium,barium, titanium (II), manganese (II), iron (II), cobalt (II), nickel(II), zirconium (II), and tin (II).

Examples of naphthalenethiols include 2-naphthalenethiol,1-naphthalenethiol, 2-chloro-1-naphthalenethiol,2-bromo-1-naphthalenethiol, 2-fluoro-1-naphthalenethiol,2-cyano-1-naphthalenethiol, 2-acetyl-1-naphthalenethiol,1-chloro-2-naphthalenethiol, 1-bromo-2-naphthalenethiol,1-fluoro-2-naphthalenethiol, 1-cyano-2-naphthalenethiol,1-acetyl-2-naphthalenethiol, and metal salts thereof.1-naphthalenethiol, 2-naphthalenethiol, and zinc salts thereof arepreferred.

Examples of sulfenamide type organic sulfur compounds includeN-cyclohexyl-2-benzothiazole sulfenamide,N-oxydiethylene-2-benzothiazole sulfenamide, andN-t-butyl-2-benzothiazole sulfenamide. Examples of thiuram type organicsulfur compounds include tetramethylthiuram monosulfide,tetramethylthiuram disulfide, tetraethylthiuram disulfide,tetrabutylthiuram disulfide, and dipentamethylenethiuram tetrasulfide.Examples of dithiocarbamates include zinc dimethyldithiocarbamate, zincdiethyldithiocarbamate, zinc dibutyldithiocarbamate, zincethylphenyldithiocarbamate, sodium dimethyldithiocarbamate, sodiumdiethyldithiocarbamate, copper (II) dimethyldithiocarbamate, iron (III)dimethyldithiocarbamate, selenium diethyldithiocarbamate, and telluriumdiethyldithiocarbamate. Examples of thiazole type organic sulfurcompounds include 2-mercaptobenzothiazole (MBT); dibenzothiazyldisulfide (MBTS); a sodium salt, a zinc salt, a copper salt, or acyclohexylamine salt of 2-mercaptobenzothiazole;2-(2,4-dinitrophenyl)mercaptobenzothiazole; and2-(2,6-diethyl-4-morpholinothio)benzothiazole.

In light of resilience performance, the amount of the organic sulfurcompound (e) is preferably equal to or greater than 0.05 parts by weightand particularly preferably equal to or greater than 0.1 parts by weightper 100 parts by weight of the base rubber. In light of resilienceperformance, the amount is preferably equal to or less than 5.0 parts byweight and particularly preferably equal to or less than 2.0 parts byweight per 100 parts by weight of the base rubber.

For the purpose of adjusting specific gravity and the like, a filler maybe included in the core 4. Examples of suitable fillers include zincoxide, barium sulfate, calcium carbonate, and magnesium carbonate. Theamount of the filler is determined as appropriate so that the intendedspecific gravity of the core 4 is accomplished. A particularlypreferable filler is zinc oxide. Zinc oxide serves not only as aspecific gravity adjuster but also as a crosslinking activator.

According to need, an anti-aging agent, a coloring agent, a plasticizer,a dispersant, sulfur, a vulcanization accelerator, and the like areadded to the rubber composition of the core 4. Crosslinked rubber powderor synthetic resin powder may also be dispersed in the rubbercomposition.

In the core 4, the difference (Hs−H(0)) between the surface hardness Hsand the central hardness H(0) is preferably equal to or greater than 15.The difference is great. In other words, the core 4 has anouter-hard/inner-soft structure. When the core 4 is hit with a middleiron, the recoil (torsional return) is great, and thus spin issuppressed. The core 4 contributes to the flight performance of the golfball 2. In light of flight performance, the difference (Hs−H(0)) is morepreferably equal to or greater than 20 and particularly preferably equalto or greater than 25. From the standpoint that the core 4 can easily beformed, the difference (Hs−H(0)) is preferably equal to or less than 50.

The hardness H(0) at the central point of the core 4 is preferably equalto or greater than 40.0 but equal to or less than 70.0. The golf ball 2having a hardness H(0) of 40.0 or greater has excellent resilienceperformance. In this respect, the hardness H(0) is more preferably equalto or greater than 45.0 and particularly preferably equal to or greaterthan 50.0. The core 4 having a hardness H(0) of 70.0 or less can achievean outer-hard/inner-soft structure. In the golf ball 2 that includes thecore 4, spin can be suppressed. In this respect, the hardness H(0) ismore preferably equal to or less than 68.0 and particularly preferablyequal to or less than 66.0.

The hardness Hs at the surface of the core 4 is preferably equal to orgreater than 78.0 but equal to or less than 95.0. The core 4 having ahardness Hs of 78.0 or greater can achieve an outer-hard/inner-softstructure. In the golf ball 2 that includes the core 4, spin can besuppressed. In this respect, the hardness Hs is more preferably equal toor greater than 80.0 and particularly preferably equal to or greaterthan 82.0. The golf ball 2 having a hardness Hs of 95.0 or less hasexcellent durability. In this respect, the hardness Hs is morepreferably equal to or less than 93.0 and particularly preferably equalto or less than 90.0.

The core 4 has a diameter of preferably 38.0 mm or greater but 42.0 mmor less. The core 4 having a diameter of 38.0 mm or greater can achieveexcellent resilience performance of the golf ball 2. In this respect,the diameter is more preferably equal to or greater than 38.5 mm andparticularly preferably equal to or greater than 39.0 mm. In the golfball 2 that includes the core 4 having a diameter of 42.0 mm or less,the inner cover 8 and the outer cover 10 can have sufficientthicknesses. The golf ball 2 that includes the inner cover 8 and theouter cover 10 having large thicknesses have excellent durability. Inthis respect, the diameter is more preferably equal to or less than 41mm and particularly preferably equal to or less than 40 mm. The core 4may include two or more layers.

In light of feel at impact, the core 4 has an amount of compressivedeformation Dc of preferably 3.0 mm or greater and particularlypreferably 3.3 mm or greater. In light of resilience performance, theamount of compressive deformation Dc is preferably equal to or less than4.6 mm and particularly preferably equal to or less than 4.3 mm.

For the inner cover 8, a resin composition is suitably used. Examples ofthe base polymer of the resin composition include ionomer resins,styrene block-containing thermoplastic elastomers, thermoplasticpolyester elastomers, thermoplastic polyamide elastomers, andthermoplastic polyolefin elastomers.

Particularly preferable base polymers are ionomer resins. The golf ball2 that includes the inner cover 8 including an ionomer resin hasexcellent resilience performance. An ionomer resin and another resin maybe used in combination for the inner cover 8. In this case, theprincipal component of the base polymer is preferably the ionomer resin.Specifically, the proportion of the ionomer resin to the entire basepolymer is preferably equal to or greater than 50% by weight, morepreferably equal to or greater than 60% by weight, and particularlypreferably equal to or greater than 70% by weight.

Examples of preferable ionomer resins include binary copolymers formedwith an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms. A preferable binary copolymer includes 80% by weight ormore but 90% by weight or less of an α-olefin, and 10% by weight or morebut 20% by weight or less of an α,β-unsaturated carboxylic acid. Thebinary copolymer has excellent resilience performance. Examples of otherpreferable ionomer resins include ternary copolymers formed with: anα-olefin; an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms;and an α,β-unsaturated carboxylate ester having 2 to 22 carbon atoms. Apreferable ternary copolymer includes 70% by weight or more but 85% byweight or less of an α-olefin, 5% by weight or more but 30% by weight orless of an α,β-unsaturated carboxylic acid, and 1% by weight or more but25% by weight or less of an α,β-unsaturated carboxylate ester. Theternary copolymer has excellent resilience performance. For the binarycopolymers and the ternary copolymers, preferable α-olefins are ethyleneand propylene, while preferable α,β-unsaturated carboxylic acids areacrylic acid and methacrylic acid. A particularly preferable ionomerresin is a copolymer formed with ethylene and acrylic acid ormethacrylic acid.

In the binary copolymers and the ternary copolymers, some of thecarboxyl groups are neutralized with metal ions. Examples of metal ionsfor use in neutralization include sodium ion, potassium ion, lithiumion, zinc ion, calcium ion, magnesium ion, aluminum ion, and neodymiumion. The neutralization may be carried out with two or more types ofmetal ions. Particularly suitable metal ions in light of resilienceperformance and durability of the golf ball 2 are sodium ion, zinc ion,lithium ion, and magnesium ion.

Specific examples of ionomer resins include trade names “Himilan 1555”,“Himilan 1557”, “Himilan 1605”, “Himilan 1706”, “Himilan 1707”, “Himilan1856”, “Himilan 1855”, “Himilan AM7311”, “Himilan AM7315”, “HimilanAM7317”, “Himilan AM7318”, “Himilan AM7329”, and “Himilan AM7337”,“Himilan MK7320”, and “Himilan MK7329”, manufactured by Du Pont-MITSUIPOLYCHEMICALS Co., Ltd.; trade names “Surlyn 6120”, “Surlyn 6910”,“Surlyn 7930”, “Surlyn 7940”, “Surlyn 8140”, “Surlyn 8150”, “Surlyn8940”, “Surlyn 8945”, “Surlyn 9120”, “Surlyn 9150”, “Surlyn 9910”,“Surlyn 9945”, “Surlyn AD8546”, “HPF1000”, and “HPF2000”, manufacturedby E.I. du Pont de Nemours and Company; and trade names “IOTEK 7010”,“IOTEK 7030”, “IOTEK 7510”, “IOTEK 7520”, “IOTEK 8000”, and “IOTEK8030”, manufactured by ExxonMobil Chemical Corporation.

Two or more ionomer resins may be used in combination for the innercover 8. An ionomer resin neutralized with a monovalent metal ion, andan ionomer resin neutralized with a bivalent metal ion may be used incombination.

A preferable resin that can be used in combination with an ionomer resinis a styrene block-containing thermoplastic elastomer. The styreneblock-containing thermoplastic elastomer has excellent compatibilitywith ionomer resins. A resin composition including the styreneblock-containing thermoplastic elastomer has excellent fluidity.

The styrene block-containing thermoplastic elastomer includes apolystyrene block as a hard segment, and a soft segment. A typical softsegment is a diene block. Examples of diene compounds include butadiene,isoprene, 1,3-pentadiene, and 2,3-dimethyl-1,3-butadiene. Butadiene andisoprene are preferred. Two or more compounds may be used incombination.

Examples of styrene block-containing thermoplastic elastomers includestyrene-butadiene-styrene block copolymers (SBS),styrene-isoprene-styrene block copolymers (SIS),styrene-isoprene-butadiene-styrene block copolymers (SIBS), hydrogenatedSBS, hydrogenated SIS, and hydrogenated SIBS. Examples of hydrogenatedSBS include styrene-ethylene-butylene-styrene block copolymers (SEBS).Examples of hydrogenated SIS include styrene-ethylene-propylene-styreneblock copolymers (SEPS). Examples of hydrogenated SIBS includestyrene-ethylene-ethylene-propylene-styrene block copolymers (SEEPS).

In light of resilience performance of the golf ball 2, the content ofthe styrene component in the styrene block-containing thermoplasticelastomer is preferably equal to or greater than 10% by weight, morepreferably equal to or greater than 12% by weight, and particularlypreferably equal to or greater than 15% by weight. In light of feel atimpact of the golf ball 2, the content is preferably equal to or lessthan 50% by weight, more preferably equal to or less than 47% by weight,and particularly preferably equal to or less than 45% by weight.

In the present invention, styrene block-containing thermoplasticelastomers include alloys of olefin and one or more members selectedfrom the group consisting of SBS, SIS, SIBS, SEBS, SEPS, SEEPS, andhydrogenated products thereof. The olefin component in the alloy ispresumed to contribute to improvement of compatibility with ionomerresins. Use of this alloy improves the resilience performance of thegolf ball 2. An olefin having 2 to 10 carbon atoms is preferably used.Examples of suitable olefins include ethylene, propylene, butene, andpentene. Ethylene and propylene are particularly preferred.

Specific examples of polymer alloys include trade names “RabalonT3221C”, “Rabalon T3339C”, “Rabalon SJ4400N”, “Rabalon SJ5400N”,“Rabalon SJ6400N”, “Rabalon SJ7400N”, “Rabalon SJ8400N”, “RabalonSJ9400N”, and “Rabalon SR04”, manufactured by Mitsubishi ChemicalCorporation. Other specific examples of styrene block-containingthermoplastic elastomers include trade name “Epofriend A1010”manufactured by Daicel Chemical Industries, Ltd., and trade name “SeptonHG-252” manufactured by Kuraray Co., Ltd.

According to need, a coloring agent such as titanium dioxide and afluorescent pigment, a filler such as barium sulfate, a dispersant, anantioxidant, an ultraviolet absorber, a light stabilizer, a fluorescentmaterial, a fluorescent brightener, and the like are included in theresin composition of the inner cover 8 in an adequate amount. The innercover 8 may include powder of a metal with a high specific gravity.

From the standpoint that an outer-hard/inner-soft structure can beachieved in a sphere 16 consisting of the core 4 and the inner cover 8,the inner cover 8 has a hardness Hi of preferably 80 or greater, morepreferably 83 or greater, and particularly preferably 85 or greater. Inlight of feel at impact of the golf ball 2, the hardness Hi ispreferably equal to or less than 95 and particularly preferably equal toor less than 90. The hardness Hi is measured with a JIS-C type hardnessscale mounted to an automated rubber hardness measurement machine (tradename “P1”, manufactured by Kobunshi Keiki Co., Ltd.). For themeasurement, a slab that is formed by hot press and that has a thicknessof about 2 mm is used. A slab kept at 23° C. for two weeks is used forthe measurement. At the measurement, three slabs are stacked. A slabformed from the same resin composition as the resin composition of theinner cover 8 is used.

From the standpoint that an outer-hard/inner-soft structure is achievedin the sphere 16 and spin of the golf ball 2 is suppressed, the hardnessHi of the inner cover 8 is preferably greater than the surface hardnessHs of the core 4. In light of suppression of spin, the difference(Hi−Hs) between the hardness Hi and the hardness Hs is preferably equalto or greater than 1 and particularly preferably equal to or greaterthan 2. The difference (Hi−Hs) is preferably equal to or less than 5. Inthe sphere 16 having a difference (Hi−Hs) of 5 or less, the hardnesslinearly increases from its central point toward its surface. In thesphere 16 whose hardness linearly increases, the energy loss is low whenbeing hit with a middle iron.

The inner cover 8 has a thickness of preferably 0.2 mm or greater but2.0 mm or less. In the sphere 16 that includes the inner cover 8 havinga thickness of 0.2 mm or greater, an outer-hard/inner-soft structure canbe achieved. In this respect, the thickness of the inner cover 8 is morepreferably equal to or greater than 0.5 mm and particularly preferablyequal to or greater than 0.8 mm. The golf ball 2 that includes the innercover 8 having a thickness of 2.0 mm or less has excellent resilienceperformance. In this respect, the thickness is more preferably equal toor less than 1.6 mm and particularly preferably equal to or less than1.3 mm.

In light of feel at impact, the sphere 16 consisting of the core 4 andthe inner cover 8 has an amount of compressive deformation Di ofpreferably 3.2 mm or greater and particularly preferably 3.4 mm orgreater. In light of resilience performance, the amount of compressivedeformation Di is preferably equal to or less than 3.8 mm andparticularly preferably equal to or less than 3.6 mm.

For forming the inner cover 8, known methods such as injection molding,compression molding, and the like can be used.

For the outer cover 10, a resin composition is suitably used. Preferablebase polymers for use in the resin composition are ionomer resins. Thegolf ball 2 that includes the outer cover 10 including an ionomer resinhas excellent resilience performance. The ionomer resins described abovefor the inner cover 8 can be used for the outer cover 10.

An ionomer resin and another resin may be used in combination. In thiscase, in light of resilience performance, the ionomer resin is includedas the principal component of the base polymer. The proportion of theionomer resin to the entire base polymer is preferably equal to orgreater than 50% by weight, more preferably equal to or greater than 60%by weight, and particularly preferably equal to or greater than 70% byweight.

A preferable resin that can be used in combination with an ionomer resinis an ethylene-(meth)acrylic acid copolymer. The copolymer is obtainedby a copolymerization reaction of a monomer composition that containsethylene and (meth)acrylic acid. In the copolymer, some of the carboxylgroups are neutralized with metal ions. The copolymer includes 3% byweight or more but 25% by weight or less of a (meth)acrylic acidcomponent. An ethylene-(meth)acrylic acid copolymer having a polarfunctional group is particularly preferred. A specific example ofethylene-(meth)acrylic acid copolymers is trade name “NUCREL”manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.

Another preferable resin that can be used in combination with an ionomerresin is a styrene block-containing thermoplastic elastomer. The styreneblock-containing thermoplastic elastomers described above for the innercover 8 can be used for the outer cover 10.

According to need, a coloring agent such as titanium dioxide and afluorescent pigment, a filler such as barium sulfate, a dispersant, anantioxidant, an ultraviolet absorber, a light stabilizer, a fluorescentmaterial, a fluorescent brightener, and the like are included in theouter cover 10 in an adequate amount.

The outer cover 10 has a JIS-C hardness Ho of preferably 83 or greaterbut 96 or less. The golf ball 2 that includes the outer cover 10 havinga hardness Ho of 83 or greater can achieve an outer-hard/inner-softstructure. In the golf ball 2 having the outer-hard/inner-softstructure, spin is suppressed. The golf ball 2 has excellent flightperformance. In this respect, the hardness Ho is more preferably equalto or greater than 84 and particularly preferably equal to or greaterthan 85. The golf ball 2 that includes the outer cover 10 having ahardness Ho of 96 or less has excellent feel at impact. In this respect,the hardness Ho is more preferably equal to or less than 95 andparticularly preferably equal to or less than 93. The hardness Ho ismeasured by the same measurement method as that for the hardness Hi.

The outer cover 10 has a thickness of preferably 0.2 mm or greater but1.5 mm or less. The outer cover 10 having a thickness of 0.2 mm orgreater can easily be formed. In this respect, the thickness is morepreferably equal to or greater than 0.4 mm and particularly preferablyequal to or greater than 0.6 mm. In the golf ball 2 that includes theouter cover 10 having a thickness of 1.5 mm or less, spin is suppressed.In this respect, the thickness is more preferably equal to or less than1.3 mm and particularly preferably equal to or less than 1.1 mm.

For forming the outer cover 10, known methods such as injection molding,compression molding, and the like can be used. When forming the outercover 10, the dimples 12 are formed by pimples formed on the cavity faceof a mold.

The cover 6 has a total thickness of preferably 2.5 mm or less. The golfball 2 that includes the cover 6 having a total thickness of 2.5 mm orless has excellent feel at impact. In this respect, the total thicknessis more preferably equal to or less than 2.3 mm and particularlypreferably equal to or less than 2.1 mm. In light of durability of thegolf ball 2, the total thickness is preferably equal to or greater than0.3 mm, more preferably equal to or greater than 0.5 mm, andparticularly preferably equal to or greater than 0.8 mm.

The outer cover 10 has a Shore D hardness Ho greater than a Shore Dhardness Hi of the inner cover 8. The outer cover 10 can achieve anouter-hard/inner-soft structure of the golf ball 2. The golf ball 2 hasexcellent flight performance and feel at impact. The difference (Ho−Hi)is preferably equal to or greater than 2, more preferably equal to orgreater than 4, and particularly preferably equal to or greater than 6.In light of suppression of energy loss when being hit, the difference(Ho−Hi) is preferably equal to or less than 10.

In a hardness distribution curve of the golf ball 2 from the centralpoint of the core 4 to the outer cover 10, the hardness of the outercover 10 is the greatest. In the golf ball 2, spin is suppressed.

In light of feel at impact, the golf ball 2 has an amount of compressivedeformation Db of preferably 2.8 mm or greater, more preferably 2.9 mmor greater, and particularly preferably 3.0 mm or greater. In light ofresilience performance, the amount of compressive deformation Db ispreferably equal to or less than 3.6 mm, more preferably equal to orless than 3.5 mm, and particularly preferably equal to or less than 3.4mm.

At measurement of the amount of compressive deformation, first, a spheresuch as the core 4, the golf ball 2, or the like is placed on a hardplate made of metal. Next, a cylinder made of metal gradually descendstoward the sphere. The sphere, squeezed between the bottom face of thecylinder and the hard plate, becomes deformed. A migration distance ofthe cylinder, starting from the state in which an initial load of 98 Nis applied to the sphere up to the state in which a final load of 1274 Nis applied thereto, is measured.

EXAMPLES Example 1

A rubber composition was obtained by kneading 100 parts by weight of ahigh-cis polybutadiene (trade name “BR-730”, manufactured by JSRCorporation), 26 parts by weight of zinc diacrylate (trade name“Sanceler SR”, manufactured by SANSHIN CHEMICAL INDUSTRY CO., LTD.), 5parts by weight of zinc oxide, an appropriate amount of barium sulfate,0.2 parts by weight of 2-naphthalenethiol, 10 parts by weight of zincstearate, and 0.75 parts by weight of dicumyl peroxide. This rubbercomposition was placed into a mold including upper and lower mold halveseach having a hemispherical cavity, and heated at 170° C. for 25 minutesto obtain a core with a diameter of 39.1 mm. The amount of bariumsulfate was adjusted such that the weight of a golf ball is 45.4 g.

A resin composition was obtained by kneading 40 parts by weight of anionomer resin (the aforementioned “Himilan AM7337”), 40 parts by weightof another ionomer resin (the aforementioned “Himilan AM7329”), 20 partsby weight of a styrene block-containing thermoplastic elastomer (theaforementioned “Rabalon T3221C”), and 6 parts by weight of titaniumdioxide with a twin-screw kneading extruder. The core was placed into amold. The core was covered with the resin composition by injectionmolding to form an inner cover with a thickness of 1.0 mm.

A resin composition was obtained by kneading 5 parts by weight of anionomer resin (the aforementioned “Himilan AM7337”), 10 parts by weightof another ionomer resin (the aforementioned “Himilan 1555”), 55 partsby weight of still another ionomer resin (the aforementioned “HimilanAM7329”), 30 parts by weight of an ethylene-(meth)acrylic acid copolymer(trade name “NUCREL N1050H”, manufactured by Du Pont-MITSUIPOLYCHEMICALS Co., Ltd.), 3 parts by weight of titanium dioxide, and 0.2parts by weight of an ultraviolet absorber (trade name “TINUVIN 770”,manufactured by Ciba Japan K.K.) with a twin-screw kneading extruder.The sphere consisting of the core and the inner cover was placed into afinal mold having a large number of pimples on its cavity face. Thesphere was covered with the resin composition by injection molding toform an outer cover with a thickness of 0.8 mm. Dimples having a shapethat is the inverted shape of the pimples were formed on the outercover. A clear paint including a two-component curing type polyurethaneas a base material was applied to this outer cover to obtain a golf ballof Example 1 with a diameter of 42.7 mm.

Examples 2 to 13 and Comparative Examples 1 to 6

Golf balls of Examples 2 to 13 and Comparative Examples 1 to 6 wereobtained in the same manner as Example 1, except the specifications ofthe core, the inner cover, and the outer cover were as shown in Tables 7to 9 below. The composition of the core is shown in detail in Tables 1and 2 below. The compositions of the inner cover and the outer cover areshown in detail in Table 3 below. The hardness distribution of the coreis shown in Tables 4 to 6 below. The golf ball according to ComparativeExample 3 does not have an inner cover.

[Hit with Middle Iron (I#5)]

A #5-iron (trade name “XXIO”, manufactured by SRI Sports Limited, shafthardness: R, loft angle: 24°) was attached to a swing machinemanufactured by Golf Laboratories, Inc. A golf ball was hit under thecondition of a head speed of 35 m/sec. The spin rate was measuredimmediately after the hit. Further, the distance from the launch pointto the stop point was measured. The average value of data obtained by 12measurements is shown in Tables 7 to 9 below.

TABLE 1 Composition of Core (parts by weight) A C D E F M BR-730 100 100100 100 100 100 Sanceler SR 27.0 26.0 27.5 29.5 31.5 25.0 Zinc oxide 5 55 5 5 5 Barium sulfate Appropriate amount 2-naphthalene- 0.2 0.2 0.2 0.20.2 0.2 thiol Zinc stearate 0 10 20 30 40 — Aluminum — — — — — 10stearate Dicumyl 0.75 0.75 0.75 0.75 0.75 0.75 peroxide Deformation 3.863.85 3.86 3.85 3.86 3.83 Dc (mm)

TABLE 2 Composition of Core (parts by weight) G H I J K L BR-730 100 100100 100 100 100 Sanceler SR 26.5 25.5 25.0 25.5 26.0 25.5 Zinc oxide 5 55 5 5 5 Barium sulfate Appropriate amount 2-naphthalene- 0.2 0.2 0.2 0.20.2 0.2 thiol Zinc octoate — 2.5 5 — — — Zinc laurate — — — 10 — — Zincmyristate — — — — 5 10 Zinc stearate 0.5 — — — — — Dicumyl 0.75 0.750.75 0.75 0.75 0.75 peroxide Deformation 3.86 3.87 3.83 3.85 3.86 3.84Dc (mm)

The details of the compounds listed in Tables 1 and 2 are as follows.

BR-730: a high-cis polybutadiene manufactured by JSR Corporation(cis-1,4-bond content: 96% by weight, 1,2-vinyl bond content: 1.3% byweight, Mooney viscosity (ML₁₊₄(100° C.)): 55, molecular weightdistribution (Mw/Mn): 3)

Sanceler SR: zinc diacrylate manufactured by SANSHIN CHEMICAL INDUSTRYCO., LTD. (a product coated with 10% by weight of stearic acid)

Zinc oxide: trade name “Ginrei R” manufactured by Toho Zinc Co., Ltd.

Barium sulfate: trade name “Barium sulfate BD” manufactured by SakaiChemical Industry Co., Ltd.

2-naphthalenethiol: a product of Tokyo Chemical Industry Co., Ltd.

Zinc stearate: a product of Wako Pure Chemical Industries, Ltd.

Dicumyl peroxide: trade name “Percumyl D” manufactured by NOFCorporation

Zinc octoate: a product of Mitsuwa Chemicals Co., Ltd.

Zinc laurate: a product of Mitsuwa Chemicals Co., Ltd.

Zinc myristate: a product of NOF Corporation

TABLE 3 Composition of Cover (parts by weight) C1 M1 M2 M3 M4 M5 HimilanAM7337 5 51 45 40 24 26 Himilan 1555 10 — — — — — Himilan AM7329 55 4040 40 50 40 NUCREL N1050H 30 — — — — — Rabalon T3221C — 9 15 20 26 34Titanium dioxide (A220) 3 6 6 6 6 6 TINUVIN 770 0.2 — — — — — Hardness(JIS C) 92 89 87 85 83 76

TABLE 4 Hardness Distribution of Core Comp. Comp. Comp. Ex. 1 Ex. 2 Ex.1 Ex. 2 Ex. 3 Ex. 3 Ex. 13 Composition of core C C C C C C M H(0) 54.054.0 54.0 54.0 54.0 54.0 55.6 H(2.5) 59.8 59.8 59.8 59.8 59.8 59.8 60.2H(5.0) 63.0 63.0 63.0 63.0 63.0 63.0 63.9 H(7.5) 64.6 64.6 64.6 64.664.6 64.6 65.4 H(10.0) 67.0 67.0 67.0 67.0 67.0 67.0 67.1 H(12.5) 71.871.8 71.8 71.8 71.8 71.8 70.9 H(15.0) 76.0 76.0 76.0 76.0 76.0 76.0 74.8H(17.5) 79.5 79.5 79.5 79.5 79.5 79.5 77.7 Hs 83.0 83.0 83.0 83.0 83.083.0 82.3

TABLE 5 Hardness Distribution of Core Comp. Comp. Ex. 4 Ex. 5 Ex. 6 Ex.7 Ex. 4 Ex. 5 Composition of core C C D E F A H(0) 54.0 54.0 56.5 59.261.9 59.0 H(2.5) 59.8 59.8 59.7 61.5 63.2 64.5 H(5.0) 63.0 63.0 62.063.2 64.3 67.1 H(7.5) 64.6 64.6 62.8 64.0 64.3 67.7 H(10.0) 67.0 67.066.6 66.8 67.0 68.6 H(12.5) 71.8 71.8 73.7 71.0 70.4 70.6 H(15.0) 76.076.0 75.4 72.1 70.5 74.1 H(17.5) 79.5 79.5 78.2 73.0 68.5 79.0 Hs 82.582.1 81.6 79.1 70.7 83.0

TABLE 6 Hardness Distribution of Core Comp. Ex. 8 Ex. 9 Ex. 10 Ex. 11Ex. 12 Ex. 6 Composition of core H I J K L G H(0) 53.6 51.4 54.2 54.953.3 57.5 H(2.5) 58.3 57.6 58.2 59.0 58.4 63.5 H(5.0) 61.7 61.2 62.163.6 62.6 66.6 H(7.5) 65.2 63.8 64.4 67.0 65.5 68.8 H(10.0) 67.4 67.966.4 68.5 67.4 70.0 H(12.5) 71.0 73.8 71.0 70.1 71.8 71.2 H(15.0) 75.377.8 77.0 76.7 77.5 74.8 H(17.5) 80.6 82.0 80.7 80.5 81.3 78.8 Hs 84.184.9 83.3 83.4 84.5 82.9

TABLE 7 Result of Evaluation Co. Co. Ex. Ex. Ex. Co. Ex. Ex. 1 Ex. 2 1 23 Ex. 3 13 Core Composition C C C C C C M Diameter (mm) 39.1 39.1 39.139.1 39.1 39.1 39.1 Hs − H(0) 29.0 29.0 29.0 29.0 29.0 29.0 26.7 Dc (mm)3.85 3.85 3.85 3.85 3.85 3.85 3.83 Inner cover Composition M5 M4 M3 M2M1 — M2 Thickness 1.0 1.0 1.0 1.0 1.0 — 1.0 (mm) Hi (JIS C) 76.0 83.085.0 87.0 89.0 — 87.0 Di (mm) 3.57 3.55 3.55 3.55 3.55 — 3.53 Outercover Composition C1 C1 C1 C1 C1 C1 C1 Thickness 0.8 0.8 0.8 0.8 0.8 1.80.8 (mm) Ho (JIS C) 92.0 92.0 92.0 92.0 92.0 92.0 92.0 Db (mm) 3.22 3.203.20 3.20 3.20 3.25 3.18 Cover total 1.8 1.8 1.8 1.8 1.8 1.8 1.8thickness (mm) Hi − Hs −7.0 0.0 2.0 4.0 6.0 — 4.7 Ho − Hi 16.0 9.0 7.05.0 3.0 — 5.0 Spin (rpm) 3850 3820 3790 3770 3750 3830 3785 Difference−50 −80 −110 −130 −150 −70 −115 from Co. Ex. 5 Flight 150.2 150.4 151.4151.6 151.8 150.3 151.2 distance (m) Difference 0.2 0.4 1.4 1.6 1.8 0.31.2 from Co. Ex. 5

TABLE 8 Result of Evaluation Ex. Ex. Ex. Ex. Co. Co. 4 5 6 7 Ex. 4 Ex. 5Core Composition C C D E F A Diameter (mm) 38.5 37.9 39.1 39.1 39.1 39.1Hs − H(0) 28.5 28.1 25.1 19.9 8.8 24.0 Dc (mm) 3.85 3.85 3.86 3.85 3.863.86 Inner cover Composition M3 M3 M3 M3 M3 M3 Thickness 1.0 1.0 1.0 1.01.0 1.0 (mm) Hi (JIS C) 85.0 85.0 85.0 85.0 85.0 85.0 Di (mm) 3.55 3.553.56 3.60 3.66 3.56 Outer cover Composition C1 C1 C1 C1 C1 C1 Thickness1.1 1.4 0.8 0.8 0.8 0.8 (mm) Ho (JIS C) 92.0 92.0 92.0 92.0 92.0 92.0 Db(mm) 3.18 3.16 3.21 3.25 3.31 3.21 Cover total 2.1 2.4 1.8 1.8 1.8 1.8thickness (mm) Hi − Hs 1.5 2.9 3.4 5.9 14.3 2.0 Ho − Hi 7.0 7.0 7.0 7.07.0 7.0 Spin (rpm) 3795 3775 3780 3800 4000 3900 Difference −105 −125−120 −100 100 — from Co. Ex. 5 Flight 151.1 151.3 151.2 151.0 149.4150.0 distance (m) Difference 1.1 1.3 1.2 1.0 −0.6 — from Co. Ex. 5

TABLE 9 Result of Evaluation Ex. Ex. Ex. Ex. Ex. Co. 8 9 10 11 12 Ex. 6Core Composition H I J K L G Diameter (mm) 39.1 39.1 39.1 39.1 39.1 39.1Hs − H(0) 30.5 33.5 29.1 28.5 31.2 25.4 Dc (mm) 3.87 3.83 3.85 3.86 3.843.86 Inner cover Composition M1 M1 M1 M1 M1 M1 Thickness 1.0 1.0 1.0 1.01.0 1.0 (mm) Hi (JIS C) 89.0 89.0 89.0 89.0 89.0 89.0 Di (mm) 3.57 3.533.55 3.56 3.54 3.56 Outer cover Composition C1 C1 C1 C1 C1 C1 Thickness0.8 0.8 0.8 0.8 0.8 0.8 (mm) Ho (JIS C) 92.0 92.0 92.0 92.0 92.0 92.0 Db(mm) 3.22 3.18 3.20 3.21 3.19 3.21 Cover total 1.8 1.8 1.8 1.8 1.8 1.8thickness (mm) Hi − Hs 4.9 4.1 5.7 5.6 4.5 6.1 Ho − Hi 3.0 3.0 3.0 3.03.0 3.0 Spin (rpm) 3735 3710 3745 3765 3740 3850 Difference −165 −190−155 −135 −160 −50 from Co. Ex. 5 Flight 151.9 152.2 151.7 151.5 152.0150.5 distance (m) Difference 1.9 2.2 1.7 1.5 2.0 0.5 from Co. Ex. 5

As shown in Tables 7 to 9, the golf balls according to Examples haveexcellent flight performance upon a shot with a middle iron. From theresults of evaluation, advantages of the present invention are clear.

The golf ball according to the present invention can be used for playinggolf on golf courses and practicing at driving ranges. The abovedescriptions are merely for illustrative examples, and variousmodifications can be made without departing from the principles of thepresent invention.

1. A golf ball comprising a spherical core and a cover which covers thecore and has two or more layers, wherein the core is obtained by arubber composition being crosslinked, the rubber composition includes:(a) a base rubber; (b) a co-crosslinking agent; (c) a crosslinkinginitiator; and (d) a carboxylate, the co-crosslinking agent (b) is: (b1)an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; or (b2) ametal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms, an amount of the carboxylate (d) is equal to or greater than 1parts by weight but less than 40 parts by weight per 100 parts by weightof the base rubber (a), and a JIS-C hardness Hi of an innermost layer ofthe cover is greater than a JIS-C hardness Hs at a surface of the core.2. The golf ball according to claim 1, wherein the hardness Hi is equalto or greater than 80.0 but equal to or less than 95.0.
 3. The golf ballaccording to claim 1, wherein the hardness Hs is equal to or greaterthan 78.0 but equal to or less than 95.0.
 4. The golf ball according toclaim 1, wherein the rubber composition further includes an organicsulfur compound (e).
 5. The golf ball according to claim 1, wherein therubber composition includes the α,β-unsaturated carboxylic acid (b1),and the rubber composition further includes a metal compound (f).
 6. Thegolf ball according to claim 1, wherein the carboxylate (d) is a fattyacid salt.
 7. The golf ball according to claim 6, wherein thecarboxylate (d) is a salt of a saturated fatty acid.
 8. The golf ballaccording to claim 1, wherein a carbon number of a carboxylic acidcomponent of the carboxylate (d) is equal to or greater than 4 but equalto or less than
 30. 9. The golf ball according to claim 1, wherein therubber composition includes the metal salt (b2) of the α,β-unsaturatedcarboxylic acid.
 10. The golf ball according to claim 4, wherein theorganic sulfur compound (e) is a thiophenol, a polysulfide having 2 to 4sulfur atoms, a thionaphthol, a thiuram, or a metal salt thereof. 11.The golf ball according to claim 1, wherein the rubber compositionincludes 15 parts by weight or greater but 50 parts by weight or less ofthe co-crosslinking agent (b) per 100 parts by weight of the base rubber(a).
 12. The golf ball according to claim 1, wherein the rubbercomposition includes 0.2 parts by weight or greater but 5.0 parts byweight or less of the crosslinking initiator (c) per 100 parts by weightof the base rubber (a).
 13. The golf ball according to claim 4, whereinthe rubber composition includes 0.05 parts by weight or greater but 5parts by weight or less of the organic sulfur compound (e) per 100 partsby weight of the base rubber (a).
 14. The golf ball according to claim1, wherein a difference (Hs−H(0)) between the hardness Hs and a JIS-Chardness H(0) at a central point of the core is equal to or greater than15.
 15. The golf ball according to claim 1, wherein a JIS-C hardnessH(0) at a central point of the core is equal to or greater than 40.0 butequal to or less than 70.0.
 16. The golf ball according to claim 1,wherein a difference (Hi−Hs) between the hardness Hi and the hardness Hsis equal to or greater than 1 but equal to or less than
 5. 17. The golfball according to claim 1, wherein a difference (Ho−Hi) between a JIS-Chardness Ho of an outermost layer of the cover and the JIS-C hardness Hiof the innermost layer of the cover is equal to or greater than 2 butequal to or less than
 10. 18. The golf ball according to claim 1,wherein, in a hardness distribution curve from a central point of thecore to an outermost layer of the cover, a hardness of the outermostlayer is the greatest.
 19. The golf ball according to claim 1, wherein atotal thickness of the cover is equal to or less than 2.5 mm.
 20. Thegolf ball according to claim 1, wherein when distances (mm) from acentral point of the core to the surface of the core and eight points,and JIS-C hardnesses at the surface of the core and the eight points,which eight points are obtained by dividing a region from the centralpoint of the core to the surface of the core at intervals of 2.5 mm, areplotted in a graph, R² of a linear approximation curve obtained by aleast-square method is equal to or greater than 0.95.